On the subject of everyone playing DH if it's prereleased more than a few days : that's actually a good thing, tbh.
Atm, I think it's safe to say that most players are fairly bored so it'll give us something to do. I has also several benefits : players wanting to play DH as their main char will have some time to level their professions (and generally prepare their DH for Legion) ; those sticking to their class will have some time to toy around with the new one without the pressure of the new expansion (thinking mosty about raiders, obviously ^^) ; and finally it might help people unsure about what to play if they want to pursue with the DH, even if the 100 talents are limited (still, it probably is enough to decide).
Even if the class is slightly changed between the prepatch and the released (which is always the case for existing classes during prepatch anyway), I think it would be beneficial in all regards. I can't honnestly find a good reason, nor a downside, in letting players having say a month and a half of wod-DH.