Sometimes I leave my primary laptop at work.

Sometimes my girlfriend may want to play a SNES game while I'm using my laptop.

Sometimes I want to reinstall my laptop and don't want that to impact my gaming console.

Sometimes I may not want to bother with having to plug in my laptop in the TV and find the controller, to play 20 minutes of Dr. Mario. (I'm very lazy)

I'd rather have several cheap, interchangeable devices rather than everything from a single one :) (that applies to my laptop too; I did all of my programming and work from a 300$ netbook for a couple years, that was nice and I wasn't too heartbroken when my gf at the time spilled water on it and killed it)