They can get injured at any level, but that doesn't mean that you shouldn't play them as much as possible. But you're right, the earlier that you promote them to the ML level and start their clock, the earlier that they will go through arb and then become free agents. I very rarely promote players to the majors any earlier than after at least 2 or3 professional seasons. I will get them up to AAA as soon as I can, though, as long as they will still start.