MM is generally very very good in EN.
I've only done 3 bosses so far but the only one of those where I suppose you could make an argument for BM is MAYBE Ilgynoth so you can beast cleave the ichors down. The thing is though, MM is still good at that with MS and barrage, and for everything else in the fight it's also really good - cleaving multiple, spread tentacles etc.
I have heard that BM pulls level with MM in single target when you get to about 26 points. If this really is the case then I could see potential on pure ST fights obviously but also fights with alot of movement. I just don't buy the argument that "good players don't need to worry about movement as they plan around it". It's a flawed argument imo as, on progression, alot of movement isn't predictable as you are still learning the encounter. In addition, even if you plan around movement, you will still lose abit of DPS any time you could be casting aimed shot but aren't. So yeah, if BM at 26 actually pulls level with / ahead of MM for single target then it will likely see a hell of alot more use.