It's more about portability, really. I have a good dozen (maybe 2 dozen, I haven't counted recently, hee) projects on the go right now and in the mornings before I head out I'll grab the one that says "take me!" the loudest to work on when I'm sitting around somewhere. Plus when I roll my own balls they're flat bottom center pulls, those suckers don't roll.
I've also been able to eliminate any knots in the yarn this way too -- hit a knot, thats the end of the ball.