I haven't really studied the trailers or anythign so I'm going off a vague idea but I would think crawling would be used to mix up combat. Think about it, let's say, Snake vs Link. Snake and Link are rather close together and Snake knows that Link is about to pull off a basic A A Grab sequence, so what does he do? He can either go prone for the first A and then possibly do a prone attack or he could shield the first A, go prone on the second A and then attack from there, or since he knows that the Link will A Grab if he shields the first A he can shield the first A, go prone (avoiding the grab) and then come up with an attack.
In my opinion, crawling/being prone will be another way to mix up combat. Though I wonder if like in Soul Caliber 2 and such if they'll either have rising attacks (meaning you go prone, and as you rise to standing position you execute a special move) or if they will have an assortment of prone-position attacks.
Another reason why I think crawling/prone is going to be used to mix up combat is to add another level of complexity to combat. Back to my Snake vs Link, let's say Link learns that Snake will always do Shield A, prone to miss grab, rising attack so now Link realizes a way to counter it. Link waits for Snake to come close and expect an A A Grab sequence, and right after Link does his first A (so Snake shields) he SH's and d-air's right onto now prone Snake's back.
