If you're getting collectors editions, I can see the preference to preorder. For everything else, there is practically 0 incentive (and no, an extra weapon is not an incentive). Sure maybe a % off is nice, but with the way prices come down, it's really STILL not needed. It's an outdated practice that really has no true justification in the digital download world. It's kinda like paying too much into taxes, giving them interest free money of yours to spend until return time, instead of putting that extra money away and getting interest for yourself.
It is also NOT showing support for anything...not sure where that notion came from. They are releasing the game on budgets and time frames, and no amount of preorders is going to change what the finished product looks like. This is not the same as kickstarters.