First of all, there is no bonus for people who pre purchase, so there isn't really a reason to.
And by pre purchasing you signal to the developer that you don't care how the reviews are and how many bugs there are when the game is released. This is one of the reasons that there have been a lot of games (including aoe2hd) that are released in a nearly unplayable state, but still make a lot of money.
Thank you! I imagined something like that.