Whenever I go into a game store they always say "Pre-order it now so you can have your copy the day it's released" and I look at them and say "Dude it's not that serious. I'm sure this game will be out everywhere."
GTAIV for example, I'm sure there will be more than enough copies on 4/29/08. I don't think publishing companies have the issue of shortage for games, now consoles are another thing.
Aside from the fact that the you may save $5 bucks on the game if you pre-order it. Why else should this be done?