Well, a single MM on a fast vehicle, even a BS 3 one, is a threat few opponents can afford to ignore. A single AP1 penetrating hit is usually enough to leave a vehicle crippled. Would you park a Land Raider within 24' of a weapon that stands a significant chance of destroying it?Against tanks, it is still a poor choice with only a single Bs3 shot that can harm any armour value.
The hull MM is a versatile weapon that makes the BW even better at area denial, whereas a HF makes your tank killier at the expense of tactical opportunities. Keep in mind that if there are no enemy infantry units within 12 inches of the BW, his template weapons are all but useless, and a single immobilized result is all that it takes to neutralize it once and for all.