I put the extra point into Tact mastery because the other options are 1/3 imp HS (which we can rarely do), 1/2 imp shield bash (bosses and most raid mobs are immune to silence) or 1/3 imp revenge which - same as silence - not effective outside of some 20 man trash and 5 mans which are cake.
While you only need 10 rage for mocking blow, i stance and have 15 rage, i can mocking blow and swap back before a white hit (not having to wait for it and take more damage and white hit giving less threat due to stance). Swapping back to def I then have 5 rage still for revenge.
I typically try to use a shield block prior to a stance dance to avoid being crushed during the stance dance, so revenge will almost ALWAYS be ready when I dance back to defensive.
Also while running 5 mans, with 25 rage, I'll pop shield block to put me at 15, stance-dance to Zerk stance (time it to where a white hit lands right after the stance) and can bust off a WW, zerk-rage then stance back to defensive, then revenge.
It gives a bigger window for stance dancing w/o losing any rage which we know while tanking is hard to come by.
Certain fights that have fear mechanic, and only have 1 Fear ward or even none (WTF human priest) you can't manage your rage as well for timing the incoming fear and may have to stance with 20+ rage to avoid a fear and boss turning / mob running lose, I'd like to keep as much rage as possible to get a bit more burst threat when moving back to defensive stance.
Hope that answers your question.