Online leaderboards and achievements are a bit of an unnecessary mess when it comes to mobile phones - not least on iOS, where you quite frequently end up logging on to both Game Center and Openfeint at the same time. Sometimes Crystal is involved as well, or instead, just to add to the fun.
Apple could certainly do with tidying a few things up, no doubt, but one thing that Game Center provides that generally goes unremarked is a very quick means of seeing how well certain games are selling.
When you notice that you're ranked 1,103,583 out of 1,577,591 in Tiny Tower, for example, it doesn't just illustrate how much you suck, but that an awful lot more people have downloaded that than the piddling 81,576 that have registered their Zoo Keeper DX scores. Angry Birds, predictably, manages 14.7 million registered scores, while the acclaimed Magnetic Billiards is currently at a miserable 13,514.