Why do people preorder anyway ? What if they dont like the game ?

Its not like the game will go away or anything after release.. The DLC and stuff are always negligible
Yes. This could potentially be the worst Deus Ex ever, who knows. But you will have already committed to it since you have already pre-ordered. But even if the game is good, the fanfare about pre-ordering it is a turn off and the rewards are pretty meh.
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Also, according to PC gamer:
The only way Square could reasonably guarantee an early release date is if the date it announced is intentionally pushed back from the actual ready-to-go date. The "you won't get this content if you don't preorder" threat (which, let's face it, is what this is) rings hollow too, since all this bonus stuff is already guaranteed for the retail collector's edition, which "will include all the in-game content and rewards from Augment Your Pre-order."
Source: The new Deus Ex may get an early release, but only if enough people pre-order - PC Gamer