I've played Combat til BM before I succumbed to respec-ing as a Watchman.
As combat, I found it easier to stick on 1 target due to the multiple roots that you have. However, this still does not make up for the less contact time that the Watchman needs to apply DoTs.
Some problems with Combat in PvP especially is that the Ataru proc doesn't always hit before target runs out of range. Also, MS frequently triggers the animation but doesn't actually hit the target. This results in a self-imposed root. When it hits the root on target works as intended, but this can however be interrupted or knockback.
As watchman I'm doing at least 30% more dmg than I ever did as Combat, and I've only just started out as Watchman. (This means unfamiliar keybinds and frequent frantic hitting of wrong keys...)