I wanted one too. And just this sunday got to use one for a match. It was a full metal M249 and at first I joked around wanting to carry 2 of them but just running around with 1 I've gained a new respect for that gun. IT'S HEAVY! Easily one of the heaviest I've gotten the pleasure of using.
It's safe to say I never want to own one now lol. Well maybe if I can turn one into a back mounted gat...
Like everyone else said, supression and support role. We did attack and defend and a team member had his M249 while defending and would literally just pour BB's out onto the advancing forces keeping them back.