If Bungie put in Optional MM they could also put in filters that can help fit you into a group that has the best chance of being a match for you. They could put in a MM preference page that would allow you to select Mic's Only, or perhaps Warlocks only, or lowest/highest level to play with. They could possibly even match you with players that have certain weapons. In fact they could even check all of these things far faster than an LFG site and if you want statistical based MM, nobody has more statistics about player capabilities than Bungie does for it's Destiny players (and they are more accurate than any player entered statistics). The best part is that it could be optional. So you could still bring in your own team if you wanted to use an external LFG.