At real cap games (i.e. the # of BBs in a mag is "real" to what real steel would be) and ammo limited games (i.e. the total number of BBs you can have with you regardless of what capacity mags you have).....SAW/LMG gunners can really swing the tide of things. They are then usually allowed more ammo (total and at any given time) than rifleman loads.
It would suck if at such games EVERYONE came out to such a game with a SAW/LMG...but that tends not to happen.
They work great when combined with riflemen. They work great for stitching an area while others move up (if every just holes up and snipes pot shots then they quickly lose their effect), laying in enough fire to flush out a position (i.e. to displace an opponent so your buddies can get the drop on them). They bring sustained firepower (even if they're shooting the same ammo/same FPS as rifles)...by virtue of their larger ammo capacity.
All of the above works for ammo cap/real cap stuff....when every rifle on the field has 300 rounds on tap or each guy can carry 3000 rounds....it doesn't make that much of a difference. Then you'd be better off designating a couple of guys to be "squad gunners", almost regardless of the rifles they have in their hands.
The common thing in all this...is that any use of them works best when teamed up with a couple of other guys. Work together and you'll be able to really exert maximum pressure and kick ass. With rare exception a SAW gunner on their own is going to get chewed up as fast as a lone rifleman or anyone else.