In my opinion, it isn't the best method to determine the capability of students. The way I see it, is that mocks aren't reliable. Not 'wrong' per se, but flawed. For example, let's have A, B and C (students). A cheats on their mocks and gets an A*. B revised and gained an A. C didn't revise (due to whatever reasons you want to implement) and got a U. However, C is the 'highest' achieving student of the three. They've consistent got A's and A*'s. Yet, in the mocks got a U. You see what I mean? It's flawed.