Doesn't really matter to me either way. I don't think of this as a "physics" game so much as a "puzzle" game. There's a given set of rules to work with and I design based on what I observe. I don't expect, or really desire, realism. It's immaterial. Whatever the ball does is just a factor in how I go about solving a board. I don't have any personal attachment to an arbitrary function choice.
*edit* Furthermore, thinking about this, I actually like that the ball keeps rolling. If you incorporated some sort of friction coefficient to make it stop on level ground, it would make champion solutions that much cheaper. Currently you have to put a little bit of thought into how you'll stop the ball.