Unless you do nothing but miss, a LMG can kill anyone before they can use their super or get in range unless its a Golden Gun or Voidwalker who sees you first. Of course most of this depends on what gametype you're playing as well. In Trials I found Machineguns to have less use because you need that quick instant kill ability of the rockets. Control you can defend points easier with LMGs. IMO of course.
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Rockets can let you kill the other teams heavy users easier negating their heavy. Fist of havoc and arcblade will kill your lmg quickly since 90% of these maps are cqc. If you stand in an open area you can get sniped, ranged by scouts/pulses, or grenaded much easier. Titan bubble completely negates all heavy. The only super that lmg is gonna stop is radiance and even then you'll still get hit by a grenade.