This never made sense to me. Using a little side doesn't lock you in to using exactly the spin you anticipate. If you can't find/hit center ball accurately and consistently, you can't find/hit a little side (or however much you anticipate the effects of) accurately and consistently either.
If you're anticipating using a half tip of outside, for example, you run the risk of hitting with a full tip of outside or actual center ball. Both of which could cause a miss.
I know that a lot of people do this, so I assume there's merit to it. But it doesn't make sense to me.