MRMs weren't around in 3015.
You make use with what you have it you have it as a weapon. Like I said it is not meant for that but if need be they come in handy.
From a tactical stand point it doesn't hurt to have at least one long range weapon on hand.
you are assuming that for what ever reason you ahve the choice to equip SRMs or what not.
You have to look @ this in the right context.
3015 is during the 4th Succession War. Technology is on the verge of being non existent along the front lines of the Draconis/Davion front. Salvage was for the most part the only way units could equip/be supplied.