Aura Mastery useless in PvE, intended?

Why not simply change the tooltip to state "up to 10 seconds" and make the Arena duration 6 seconds?

You say you don't like to have different rulesets for PvE/PvP, but effect durations have ample precedent.

Six seconds is hardly worth it for a PvE healer using it to temporarily mitigate AoE magical damage.