if nothing else it gets you out of the same match-up cycle that everyone else is stuck in.
Easy way to check this - go into social and as the (yet unnamed) player count goes up go into xbl 'frields' list of recent players. Here you can see GT's before they show under MM. (depending on who shows - you may want to exit mm at this point).
If/when it cycles do the same thing and note the same GT's back being MM. Best to leave this match-up set and get in with another group.
Why bungie didnt go with an opt in/out system, and let you join games already in progress - never know, especially in social. Now that numbers are down it is painful to wait for a full team, especially in th big playlists.