You never hide behind a mobile Warthog for cover for two main reasons - getting squished by it whenever it gets effected by an outside force and the fact that it has a respawn timer. A static piece of scenery that uses the Warthog model would be OK. I remember one time when someone and I were having a shootout, and he hid behind a Warthog that had been there for a while to let his shields recharge. The Warthog respawned, leaving him without any cover, and I easily dispatched him.