Personally I hate performance reviews, but if a formal system doesn't exist people will demand something. In the absence of a system there can be appearances of favoritism and, if the managers aren't disciplined, things like raises and promotions can end up being ad-hoc and random.

That said a performance review system doesn't necessarily completely cure favoritism but it at least leaves a paper trail. It should also force management into thinking about how to structure raises and promotions in a way that they're more evenly and rationally applied.