Yeah i love Watchman, definitely the most fun spec. Combat has nice big numbers but i don't think it is as useful in raids tbh.
Tanks changed me. I don't know if it was for better or worse.
There's a couple of reasons I go Carnage over Annihilation.
Theoretically, the two specs do nearly identical damage. Enough so that the major reason to choose one over the other is really personal preference. As Kitchen said, I prefer Carnage's play style and I'm better at it to the tune of about 100 DPS in dummy parsing.
Beyond that, I feel that the current crop of raids favors burst damage and no spec in the game really matches the controlled burst a Carnage marauder provides aside from Rage (and then only when 3+ targets are grouped).