For years I have kept my work on external hard drives. Oddly, my reasons back then weren't as important as they are today. Anyway, I've been asked a few times in the last few days,,, "what's the importance of doing this?"
For me, it began to make a lot of sense because I was upgrading machines nearly every 18 months or so. Of course, with every upgrade the amount of "work" data grew larger and larger. And, as laptops became more powerful, allowing me to work from a docked computing solution at home, it made even more sense to keep my work on external drives.
Look at your computer and consider what you have. You have tools and you have the product of those tools... your work. So, if you view the computer as a tool box, it makes sense to keep it just for tools... or applications.
Now, when I get a new laptop, I'll either migrate using the migration tool that comes with a new Mac... or I'll just install the programs I'm currently using. Typically, I take the path of least resistance and just migrate.
Once that's complete (usually about two hours), I plug my externals in and I'm immediately up and running and back to work with everything located where it's always been.
Currently, (and I know this is in a journal post here somewhere) I'm running a 250GB drive on my MacBook Pro. On my desk is a 23" Cinema Display, keyboard, mouse and six daisy chained firewire 800 Lacie drives. Seven if you include the small 1TB portable I use on the road.
Aperture Libraries (see link below for article on Multiple Libraries in Aperture) reside on the external desktop drives while a "current-work" Library resides on the small portable. That Library will contain current projects, while a true working copy of it resides at home on the desktop drives. But when I'm traveling, that's the drive I use and whatever new Project gets created while I'm away, is immediately transferred to the home library upon my return.
The main benefit to all of this is organization. Your work is your primary and main asset. Applications can be replaced, your work can not. There's a lot more piece of mind (at least for me) in this process and ultimately, you can't keep it all on board anyway. At some point you're going find yourself deciding what you'll move off and what you'll keep. Why not move it off in advance and have the organizational control?
And with a portable drive containing "current-work"... heck, you can hook it up to any machine, anywhere.
Keeping your laptop or desktop drives clean doesn't hurt either. I mean, who among us has never run out of hard disk space? It's not a matter of if, it's a matter of when. Don't forget, your laptop uses free space to run efficiently. Sooner or later, shortage of space will rear its ugly head.
Let's face it, hard drive space is CHEAP. You can get 500GB portables for well under $200.
Give it some thought.