First ingame preorder bonuses is an very bad business practice. In short its an way to give people who preorder an benefit, yes this helps more people buying the game, its an very huge help if the game is bad.
Second in ESO its not an huge issue, an very large amount of players has been in the closed beta and decided if they want to play or not.
Last it will be an open beta so unless they close the window before the open beta anybody would be able to preorder if they like the game.
Giving preorders a day or two head start might be an idea to spread out the players a bit. Yes its an benefit but someone who play 6 hour day will bypass the one who does 2 a day anyway.
However because of the megaserver the starting zones will not be impossible crowded like they tend to be on MMO launches.