...and a magsafe port, remember.
So, without external hubs or multiport dongles, a 2015 iMac can support a charger, a second display (HDMI), a fast hard drive (TB 2), an external mouse/keyboard (lets assume they share a USB dongle) and still leave you with a slot for a USB memory stick and a SD card (...there's a nice range of SD cards that fit flush and can be left in semi-permanently, ideal for e.g. shunting your media library or other big, static files off the expensive, fast SSD) and a spare TB2 port for a high-speed device or display - maybe an ethernet dongle if (like me) your workplace's wifi is unreliable and totally outside your control.
On a post-2016 MacBook Pro:
- Charger - scratch one USB-C port
NB: 12" MacBook owners stop reading now - even the MB Air did better than this...
- Keyboard/mouse (scratch USB Port #2 - oh, and your kb/mouse's USB-2 mini-dongle that used to only stick out 1/4" and could be left in place is now sticking out on a USB-C-to-USB-2 dongle)
NB: non-TB 2016 MBP owners, stop reading now and buy a dock. Make that two if you want to commute from home to work....and, that's all folks - the 2015 still has a TB2 and/or a USB-3 in hand, but you're out and its time to buy a dock. Maybe two if you don't want to shuttle stuff from home to work that you didn't need before. Oh, and its off to the landfill for any spare MacSafe 2 chargers that you've accumulated over the last 6 years so you could leave one at work (BTW, the new USB-C chargers still cost as much, but don't come with the charge cable and extension main cord that you used to get - you could get a 3rd party charger but few people are making ones with the oomph to charge a 15" MBP - get a cheap one if you feel lucky and live in a brick-built house).
- External display (scratch USB-C port #3 - oh, and read up on some of the troubles people have had with USB-C to HDMI/DisplayPort adapter cables esp. with the 2017 models)
- External HD (scratch USB-C port #4, unless you pay a hefty premium for a HD with TB3 through)
End of the world? No - but where a TB dock at home and work for single-port docking was a luxury solution to the first-worls problem of having to plug in a handful of cables every morning, now its a must-have... and, yes, using a laptop to shuttle from home to work, with a large display, keyboard and mouse on each desk is a not uncommon way of working.
So a computer that costs substantially more than its predecessor is (a) only incrementally faster - if at all thanks to extra thermal throttling and (b) worse as a "desktop replacement".
Of course, if you want to connect 4 Thunderbolt 3 devices you're in bandwidth heaven.... assuming your thermally-throttled i7 and ~meh mobile-class GPU can do anything with that much data...
Why wouldn't you? OK, if its mission-critical then maybe you've got a spare in the drawer along with the floppy drive, optical drive and other stuff that you no longer carry around daily...
...an eGPU which immediately obsoletes the sealed-in, no external input display in your iMac (oh and we're assuming that GPUs don't actually need the 16 lanes of direct-to-CPU bandwidth that the internal ones enjoy... now that may be true...)
Right, now add an external synth or two, a keyboard (plus any non-music stuff you have connected), and wham, you're out of USB ports and if you use a hub you've got logic moaning about changing midi devices every time you sleep. OK, so no laptop will have that many ports, but a desktop PC will typically have have 2 on the front, 6 on the back and be able to accept a cheap PCIe card with even more.
C/f iMac - 4 USB 3, 2xUSB-C but scratch one of both of the latter if you want an external display, TB3 hard drive etc. Again - first world problems but we're paying a hefty premium for these systems.
At work, I use a 2015 15" MacBook Pro (top of the line 2.8Ghz, R9 M370X GPU and 1TB of storage). I use two external Dell P2715Q 4K displays, one connected directly to the TB2 port, the other connected to an El Gato TB2 dock. My wired Apple keyboard plugs into the Dock and I use a Bluetooth Magic Mouse and Magic Trackpad. I would be using a Magic Keyboard with numeric keypad, but it's not time to replace the keyboard yet.
Without the TB2 Dock, I would only have one USB 3.0 free for use (keyobard plugged into the left USB 3.0 port). I only use flash drives sporadically and I found a dual USB-A/USB-C from Sandisk that works wonderfully.
Using the same dock with my personal 2016 MacBook Pro, I use the Dock with a TB3>TB2 adapter from Apple and a Plugable USB-C to DisplayPort cable, again 4K@60Hz for both. Power is USB-C and 1 port left over for USB devices. I don't miss the MagSafe port, although it was a great invention. Stuff changes, USB-C Power Delivery is and will be more flexible than MagSafe ever thought of being.
I don't use the HDMI port because the 2015 MacBook Pro doesn't support 4K@60Hz, as it's stuck at HDMI 1.4 and so I use DisplayPort for every monitor I have at work and at home, including our HDMI projector in the conference room (dongle).
I have used the SD Card reader from time to time for my Zoom H6 or my m4/3 camera. Its convenient, but not essential to my day in/day out. If I was a professional photographer, I have a couple of options, assuming my camera used SDXC cards. Shoot tethered, connect via a USB-C cable to whatever the camera has on the other end (usually a USB 2.0 or 3.0 Micro-B) or use a dedicated card reader, which many photographers buy anyways.
At home, I use one external display, a Dell P2415Q, so I have a USB-C to DP cable, one USB-C for the Scarlett, one USB-C for power and one USB-C for my USB external hard drives that I purchased a USB-C to USB 3.0 Type Mini-B cable from Amazon for $8.99 each (I bought two). Now I can hook up my Seagate Backup Plus Hub drive for Time Machine, my Transcend card reader or my WD My Passport drive for a Super Duper backup. I don't need an "expensive" Thunderbolt 3 drive, I can always use cheap spinners or I can purchase a superfast Samsung T5 portable SSD or a SanDisk Extreme 900 with USB-C 3.1 Type 2 and get 10Gbps, you get 5Gbps.
At home, I have been all Bluetooth for a while, bought a Magic Keyboard with numeric keypad the day it was announced, Magic TrackPad 2 and a battery powered Magic Mouse.
I considered a Thunderbolt Dock a necessity the day I purchased a 2012 15" MacBook Pro, because there really weren't any cheap Thunderbolt peripherals at that time, which never did get better and I needed more USB ports. At least with Thunderbolt 3, all I need is the right cable and I can have either 4 USB-C ports, 4 Thunderbolt 3 ports or any combo inbetween without a Dock.
True, an eGPU might not have x16 lanes like the iMac or the x8 lanes the MacBook Pro has, but the x4 lanes it does have access to seem to very helpful to those who need it. I don't game on my Mac, I have an Xbox One for that. I will be VERY appreciatve of access to an eGPU, should I want to move to a 5K or an 8K display and not burden the internal GPU. If I ever get proficient enough in Davinci Resolve, and need the extra horsepower, I can add up to 3 eGPUs without needing to build a PC and deal with Windows 10 as my daily driver.
I don't notice my 2016 MacBook Pro throttling back as much as I notice my 2015 spinning up the fans all the way to the max doing the same workload. Both are on stands to maximize airflow under the laptop. I have used cheap stands with USB fans and I have used expensive, trendy aluminum stands for every laptop I have owned since 2008. A stand is essential for airflow and ergonomics, it is not a luxury.
You can't blame anyone but Intel for the lack of significant performance bumps from the 2015 to 2016 MacBook Pros. Once Apple upgrades the MacBook Pro to the 6-core H-series CPUs, we may see some nice performance gains. I suspect it will be closer to October before that happens. Asus just announced its new ZenBook Pro with 6-core CPUs at Computex 2018, but no on sale date and no pricing was given.
Any thin and light laptop, including the 2015 MacBook Pro is going to have to make due with a mobile-class GPU unless you like lugging around something thicker and heavier, along with the beefier power supply, becasue you aren't getting more than about 2-3 hours with a 1060, a 1070 or 1080 onboard.
I can add a MIDI synth to my Scarlett interface or if I really need more ports, I can always add an external PCIe box to a 2015 15" MacBook Pro or a 2016 15" MacBook Pro and put in a PCIe card with USB 3.0 Type-A ports. The nice thing about the 2016/2017 MacBook Pro is that the link between the PCIe box and the Thunderbolt port is twice the bandwidth. Also, because Thunderbolt 3/USB-C can deliver power, either the eGPU or the PCIe box will charge my MacBook Pro and I can reclaim a TB3/USB-C port for other uses.
Please don't try to change the subject by mentioning a PC, as it's not germane to the argument unless you want to do your work in Windows. If I really need more USB ports, I have a bevy of options.
To each his own. I think the 2015 MBP is a great laptop, but I am not going to miss it when the next lease at work is started and we move to TB MacBook Pros.