I played my two BHs as PT and Merc. I liked it as long as it lasted ( follow was still broke and i started soloing Lyonheart and left my PT behind at lvl25ish ). But that was a PvE desision. For PvE i think you will like having heals for some hard mobs and for doing heroic quests. I'm not sure about PvP ( WZs ) I'm sure two of the same thing would be better/easier to control.