Tiktakkat nailed it, because a perfectly balanced system ends up pretty bland when there's no difference between options. Introducing some power creep, or simply leaving it entirely unbalanced (Marvel VS Capcom 2, anyone?) is a way to sell product, so long as the balance is semi-maintained. 3.5 sold well without even realizing the power gap though. 4E is more genre aware about this, and seeks to strike a balance inside itself (hence why Orbizards aren't getting nerfed; they're powerful, but have a weakness that prevents them from being universally used).