I've had this and it's a right pain!For injury protection. If you're going to dive around then you need paintball knee pads, elbow pads and possibly wrist protection
If you are going to crawl a lot then protect your elbows and knees from this:
http://en.m.wikipedia.org/wiki/Olecranon_bursitis
I wouldn't play without knee pads these days, especially on an astroturf pitch... defo these before elbow pads (I wear both though, along with heavily padded slide shorts and a bounce vest)