I will also have to agree that prone is absent for a good reason.
The crouching works wonders for obscuring you and others. Prone would seriously make you invisible. I think the camera view and bullets must fire from the tiptop of your head because it is very easy to get sniped by someone crouching where all you see is the top of their head, it works like prone would already.
Dynamic craters combined with prone would definitely be bad news.