I have to agree that MMLs feel a bit weak. Not because their dps is is so bad but because tmd is so effective against them. One tmd can certainly block one mml shooting at it. Except cybran but the missile split is rather a gimmick. and a tmd is only 280 mass vs 200 mass for the mml. I am very sure it is effective to block unsynced mmls with tmd. Synced mmls might be more efficient than tmd. But then you still have to break through a shield. I nearly never use them to break through a fire base. But they are great at stopping pd creeps. 3 MMLs make pd creeping with a t2 acu nearly impossible. For breaking firebases I would rather suggest t3 mobile arty, as it has only shields to fight against and it is more dps/mass efficient than t2 stationary arty.
But this discussion is only happening again because people play lame chokepoint maps where building firebases is viable. On more open maps you just can ignore the firebase and destroy your opponents eco and deal with the firebase once you are so far ahead it doesn't even matter anymore.