I like to do things just because I can, not necessarily because I have a need to...there is no real reason to run (k)ubuntu on the N900, but I do it anyway because I can. Same thing with x-plane...Sure, I have the desktop version on my computer, but there's something very satisfying about getting it running in PreEnv (not to mention the ability to fly a plane while flying in a plane - I took my N900 on my last flight for that purpose, but also to watch a few movies, experiment with GPS [I found you can get a fix at 30,000 feet by pressing it up against the window, but it's unreliable], test ideas in python when they come to me, and work on a sketch of my cat in mypaint).
I recently set up port forwarding on my home desktop; now when I'm on the train I can mount my home folders via sshfs and stream music - to the default music player as well as VLC. I used my N900 for internet at home because its 3G connection was faster than our dial-up, for all operating systems (Mac included - I have yet to get OSX tethering working with an Android phone, but with Maemo it works right out of the box!)
For a few days at work I didn't have a computer to work at, so I used the tv-out and extkeyboard to make the N900 into a miniature desktop, which seemed to run better than the netbook I have now. I would still do that but we don't have the monitor with RCA anymore.