My guess is he's a redundant piece right now - Pryor is more of a pass-rushing DT, while Brace is a big run-stuffer like Wilfork - only problem is he's just like Wilfork, meaning we have one of the best run-stuffing DTs in the league right now - we should keep in mind that while the other rookies are contributing more so far, no one else is playing at the same position as a pro-bowler (except vollmer, but that's because Light's hurt.)
Keep in mind, in this scenario, I'm saying Wilfork and Brace are "battling" for one position, Pryor and Wright for the other - if that's the case, it makes sense for Pryor to see the field way more often than Brace.