Some suggested strategies based on a lot of trial and error on my part:
(1) Best MG for play against "real people" -- MG240, close second M60. Both fire 7.62 rounds, need fewer to hit to make a kill.
(2) Best MG for play against bots (co-op) -- M249 SAW. Bots stand still or move slowly, having high rate of fire and more ammo useful in co-op missions. Also Bots "cower" more than real people will when under heavy fire, so you can actually use MG for suppression.
(3) MG's make good overwatch weapon. Put yourself on a hill by a base -- yours (defense) or theirs (offense), with claymores out on either side, just like you were a sniper. On defense, MG is often a strong defense against sudden rushes by enemy on foot or in open vehicle. On offense, MG straffing base right before your team launches an attack can be useful.
(4) If you carry an MG, don't carry any secondary weapon and only carry a few grenades, maybe two frags and one flash. Otherwise you'll be so heavy that if you have to move, you're an easy target.
(5) MGs have far less usefulness in live battles on Escalation than in the original JO. Too many protected vehicles in Escalation, not nearly as easy to find a group of people in relatively close proximity (a gunner's dream).
(6) MG can be used for close quarter work -- clear and move -- IF you don't have teammates right there with you in a "friendly fire on" game. Gunner goes into building/base first, doesn't have to worry about changing clips at critical moment due to number of rounds immediately available to any MG.
(7) Clear and move in friendly fire off map is REALLY easier with MG.