What annoys me is that if you can build a MML-7 and an MML-9, you should also be able to build SRM-8s and maybe even SRM-10s. Where are they?
But yes, MMLs are replacements for units that use both SRMs and LRMs because you really can't use both at the same time anyway because of the LRM's minimum ranges. Consider
MML-5, 3 tons, 3 crits
LRM-5, 2 tons, 2 crits
SRM-4, 2 tons, 1 crit
SRM-6, 3 tons, 2 crits
No matter which LRM/SRM combo you use, the MML winds up saving you tonnage. My only gripe is that the tube count only goes up to 9 and no higher, so they're not really good as a whole sale replacement for one weapon type or the other.