The reason they do this is explained in "Mazer in Prison". Mazer isn't that level of a commander. He's more than competent, but only had to beat the buggers once. Whoever ended up leading the third invasion would have to beat the buggers every time. So, battle school is created to screen every human at as young an age as possible, find the smart ones, and send them through training from hell in order to build up the mental and physical strength necessary to be in top mental shape for months on end. In short, they don't ride on everything on Ender for no reason, they ride everything on Ender because that's what he was trained for.