For a desktop computer, 4 USB ports is not enough. I got a Toshiba USB Hub for laptop for around $7 .. works just fine.
It is said a USB hub with dedicated power would be better, but my suggestion is you should use USB hub for device which use low powered USB, I use my hub for printer, scanner, xbox wired, iPod and flash drive sometimes.
While the iMac's USB port can be used for more "important" with high speed needed such as Time Machine backup, external HDDs
I think 4 USB is just right. I have all mine used up but have no need for more. Seriously, with everything going wireless, the number of interface ports are becoming a thing of the past.
I wouldn't be surprised if the number of ports began to shrink in the future, because of Thunderbolt daisychaining and an increasing number of wireless peripherpals hitting the market.
Yeah, assuming you use keyboard, trackpad or mouse from Apple. I don't use MagicMouse due to its drawbacks (you know what I mean) .. so a humble Logitech G500 wired USB mouse eat one of my precious USB port.
Surely right, USB ports could be diminished in the future, being less and less standard. Theoretically 1 port of Thunderbolt is enough for multiple device due to daisy chaining feature. But it's not actually that practical
Let's say we're gonna daisy chaining these: iMac --> External HDD --> Thunderbolt display --> External SSD
How if we're gonna need External HDD disconnected? Can it be unplugged without disconnecting display and SSD first? And that could be annoying since you could do the same thing without hassle or annoyance if only you have multiple Thunderbolt port.
It's a serial connections, right? While using USB hub, you can actually manage connections in parallel method.
I had 1 left on the back of the iMac(2011). I didn't like having to reach in back to temporarily plugging in a device. I solved my problem with this:
http://eshop.macsales.com/item/Macessity/LKEYSTAND/
I like this one, gives me 7 extra ports
Edit: maybe they have redesigned this product (cheaper components) based on the negative Amazon reviews. I have had two attached to (2) iMacs at home for the past 4 years, not a single issue.