Why not use two separate arms? Rather than always using Dual Mercies/Hacksaws/Cosmos

The reason I use dual Mercies is because a single mercy on it's own is less than just about every LMG and Carbine in this game, you need two mercies to give you a bit more firepower than the other weapons on there own.

Take the T9 Carv for example, it has a DPS of 1787 compared to the Mercy which has 1015, or 2030 using two mercies, thats only 243 DPS difference between a single T9 Carv and dual Mercies.