A lot differently than I used to. I used to have dual drives in my last two MacBooks via a data doubler in the optical bay, but those days are long gone.
Currently I'm using a 13" rMBP with a 256GB SSD. I keep all of my applications and critical data (eg. Dropbox documents/finances etc.) on the SSD. I then pair it with two external 4TB USB 3.0 desktop drives ($130-$160). One for all of my data such as my iTunes library, Aperture library, software DMGs, Logic files/loops, and more. And the second is my Time Machine backup for the SSD and storage drives that I keep in a safe offsite and backup to. Works pretty well and I get around 130 MB/s from them so it would be close to a good NAS over a gigabit connection. I was getting about 110 MB/s transferring data from a 2007 iMac (with an SSD mind you) to my MBP over our network with a 4th gen AirPort Extreme.
I'd consider a NAS but where I'm at it isn't a great option (renting old house, WiFi only). Back at my parents house though that's exactly what I'm doing. I'm having them order a Synology NAS to run with an ethernet cable straight into their safe in a protected room and use it to backup their iMac/MBP and a Windows box. They don't have much as far as big files go but lots of important ones. I'm hoping it will work well.
I do miss the days of being able to tote around 2TB of HDD plus an SSD in a 13" MacBook, but the awesome speed of these PCIe drives are incredible and a decent trade off when I can use a nice large external drive when I'm at a desk. That's usually the only time I need it anyway. If I'm out of the house and on the go I don't need every file and picture I've ever made haha.
So yes, MacBook plus external storage, plus NAS for backup of both drives should work just fine.