Being an Emmet main, (and I almost always play as him in FFA), I'll explain why most of us may stick to the M.A.W. launcher.
#1: Stage type. Stages like Franzea and Metropolis that change frantically makes things hard to use different loadouts, yet are large enough to use the long range M.A.W launcher.
#2 Mixing things up. Admit it, his basic loadout is ok, yet the maw launcher provides the best level one for his set up. Plus, would you rather some multi-level 1's or a devastating level 3? Your choice.
But I do have to admit, using his shotgun loadout and simply tapping neutral traingle and then side triangle on an unsuspecting group of people is a quick way to rack up some AP for that level 3.
I normally try to use shotgun loadout and then switch to the rocket launcher for the level 1 as it is definitely his best level 1, but if i'm getting **bleep** off by spammers i tend to just spam rocket launcher and laugh in their faces when they get **bleep** off. An eye for an eye and all that.