Just once. Sergeant Forge rolls the Arbiter's body, with armor, off the edge. In Halo 2, the Prophet claims it's been used by every abiriter to date. Although, if the armour was damaged by the fall, the Covenant may be able to retrieve and repair it. Actually, more times than that: Ripa 'Moramee's Arbiter armor in Halo Wars is different from Thel 'Vadamee's armor in Halo 2 and 3. In Halo Wars, you can use the Scarab and the Brutes, but the Covenant didn't get either of them until Halo 2.