They are being used, but the few people that do use them invest tons of money to make them work in smaller, less kicking calibers like 6mm/250 or 260 Remington. AR10s have a reputation for unreliability.
David Tubb (who dominates highpower) used a Knight SR25 for three years, had beaucoup problems trying to make it work in 260 Rem, and won only one national title in those three years. He then abandoned it and went back to kicking ass and taking names with bolt guns (Model 70s at first, then his own design TUBB 2000).
For the money you will shell out for a tricked out AR10, you are WAY better off buying or building a custom bolt gun in the same caliber. It will be lots cheaper in the long run and dead reliable. And no, a bolt action is not a handicap in rapid fire. Most of the people who claim that have never used one in competition. BTDT.