They have the shortest range in the game along with small lasers and flamers. Entirely useless in all but close brawls.
They require constant exposure to achieve their DPS potential. If you're only firing, say, half the time with MGs, then you're cutting down, say, 2 DPS to 1, as opposed to their closest energy equivalent, small lasers, which can do their damage in a short burst, go in and out of cover, another burst, and be at full DPS potential with decent timing.
An MG + a ton of ammo is 1.5t. If you figure 1 ton of ammo for every 2-3 MGs, then 6 MGs (the most any mech can mount) is going to be 5-6 tons. Meanwhile, 6 small lasers are going to be 3 tons on their own, so let's throw in 2-3 DHS to get the tonnage to equal out to 5-6 tons, then compare the weapon systems:
- Same long range, MG with a longer max range (though dropoff gets pretty brutal outside double).
- Both have 1 DPS.
- The small laser generates heat, and the MG does not (but 12-13 DHS.. 10 in the engine + 2-3.. can easily handle 6 small lasers).
- The small laser requires exposure of 1 second every 2 seconds or so.. so basically, the small laser needs to be exposed 1/3 of the time as the MG to do its full damage potential. This not only applies to dodging between buildings and such, but also for things like twisting to spread damage and juking.
- The small laser fires directly to the crosshairs; the MG requires at least a small amount of lead judgement.
- The MG has limited ammo and is worthless afterwards. The small laser only has a minor amount of heat to contend with, and heat can always be dissipated.
- Having an MG means adding something to your mech that can cause it (or a part of it) to explode earlier than its internals give out.
- I think it's also worth pointing out that any DHS you add for the small lasers can also serve as DHS for weapon systems in another bracket (for example, you could run 2 PPC and 6 small lasers and obviously have 2 distinct brackets, with the DHS you add serving as cooling for either bracket) whereas ammo added for an MG only serves as ammo added for an MG (well, and as a chance to explode. That's always nice).
I think these comparisons makes it obvious that an MG even at the same DPS as a small laser is a significantly weaker weapon system on the whole than the small laser. I would propose a DPS of somewhere from 1.25 to 1.5.. wouldn't want to go higher due to no heat and the complementary effect that could stack up with other weapon systems. Higher direct weapon tonnage used than the SLs (as you must have ammo for the MGs but don't have to add HS for the SL), the need for constant exposure for full DPS potential, limited shots, and a chance for your mech to explode should equal the MG having a higher reward than SLs in the same range bracket and the same base tonnage..