My co-tank is a BrM. 4/13 HC atm, and I'm beating him on a lot of fights. He's no slouch either, we've both ranked #1 on fights.
However, this is on heroic, which is innately higher vengeance. On things like trash or normal modes, Keg Smash/Dizzying Haze is retard-strong threat, which snags all aggro, which gives more vengeance, which gives more threat...continue ad nauseum.
On boss fights, I'm usually doing more of/all of the main tanking, while he does more of the switch/add duties. This allows me to get high veng, which we scale well with for both DPS and HPS. Likewise, as you noted, they do high dmg without much veng, so no loss for him there. Additionally, it allows him to pick up adds or whatever, as he's more mobile and has better aoe threat.
All about playing to your strengths and the strengths of your group.
But yeah, forget about threat vs. a monk on trash. Go AFK and get a beer or something.