Yeah... I used to do Picture Wars without a tablet, but you had plenty of time to zoom in and do stuff and most of the stuff you did was a lot smaller. For larger pictures, I hate using the mouse for drawing. It's just so awkward and hard to control when you want to make broad strokes.
Of course, like you said, I'm used to using the mouse for games and browsing. That's more of a "get object from point A to point B" process than "make a nice looking line". I'm sure, with practice, folks can get really good with using the mouse for art, but the tablet's easier to do it with.
I may have to give up the one I use, though. It's not mine, so its owner may come calling some day.