I used to pre-order all the time. When I was an active Amazon TURK user, I'd be getting free Amazon credit left and right, a bit of which I'd apply to game pre-orders that specifically gave $10+ back as more Amazon credit. For active users of the system, it was basically get a free game and be paid for it. This is actually one of the main reasons TURK pays out less now and you see less pre-orders with credit kickbacks.
Aside from that, I'd only pre-order games I knew I was going to enjoy, the most recent one being Costume Quest 2 and the current WoW expansion.