While 3 tons nets you up to 10 damage with MGs, the same weight can buy you one SRM-4 with up to 8 damage across four clusters with triple the range. OK, there is heat, but the range increase is significant enough to off-set the heat.
Alternatively, three tons can be used for 6 Small Lasers, for up to 12 damage for the same range as the MGs. This time the heat cost is worth considering, on the other hand, the MG ammo is explosive and it is unlikely you'll use more than half of the ammo.
Yet another alternative is 3 Medium Lasers, for up to 15 damage with triple the range.
Don't get me wrong, MGs have their uses. But unless you're expecting to fight infantry and collateral damage is not acceptable (ie no flamers), then MGs are not exactly a great choice.