Who cares? Let people use whatever the heck they want. This isn't a competition.
Maybe so, but wouldn't it be nice if the optimal weapon to use with a certain class was in fact unique to that class, as opposed to being optimal for the majority of classes?
Ultimately, I think that the answer lies in granting hefty class-specific weapon bonuses (either in the form of weight reduction, increased damage, increased RoF, increased recoil compensation, increased reload time, etc) to the different races for their respective weapons.
If you think about this, it only makes sense. There is no way that a Volus should be able to wield the Claymore or the Graal with the same effectiveness as a Krogan. Nor should a human be able to wield the Phaeston to the same effect as a Turian.
The lack of class-specific bonuses results in the vast majority of players using a only small subset of the weapon base, which is pretty lame, but understandable, since the behavior of competitive human beings converges towards optimality. And since only a small subset of the weapons yield optimal play, these weapons are used disproportionately. The Acolyte, IMO, exemplifies this point, but it is not the only culprit.
In a perfect ME3MP world, or ME:A MP as I envision it, each racial class would have class-specific weapons at the common, uncommon, rare, and ultra-rare levels that synergize near optimally with some kits in the class. Asaris should get more mileage out of the Disciple than the Wraith, Turians should do better with the Phaeston than the Harrier, etc, etc.
This problem does not have a trivial solution, but I would love to see it addressed in the next game.