Let's do some napkin math. A size 1 launcher needs reload rate 6 (59,000 RP) to fire one missile every increment. A size 12 bus that delivers ten missiles needs a reload rate of 8 (244,000 RP) to fire at an equivalent rate (slightly faster, 1 missile/4.5 seconds). The smaller launcher costs 9 wealth and 90 RP, the bigger costs 132 wealth and 1320 RP.
I'm trying to think of a use case where this MIRV would win out, and coming up short. I haven't played with any potential sensors for it but I imagine that getting the range for the resolution needed for that range might not be easy. The extra range of the bus is only really helpful if you detect incoming missiles farther than your AMMs can reach- in my experience adding excessive range to the missiles is easier than pushing out your detection range, and you'll be launching as soon as the contacts register. And even if the bus can match a single launcher for rate of fire, it does so by taking up 12 times the space- if the size 12 launcher competed on the basis of missiles per increment for an equivalent tonnage rather than a single launcher comparison, it would lose horrifically. Even at maximum launcher reload rate tech you're looking at 1 AMM/3s as opposed to 12/5.
The only sort of usage I could see maybe being practical is in a closing fight or in a pursuit where the combatant fleets are moving directly towards or away from one another, and the MIRVs are fired blind -on ballistic trajectories, so to speak- to catch any incoming salvos that happen to cross that line of space. Heavy use of sensor pickets might also make it workable.