Shotguns! Why not allow Engie's to use all?

I've been thinking about this previously, but never really cared enough (or was bored enough to bring it up in conversation). Really, an engineer has one gun due to most giving up the pistol for the wrangler (a good choice). Why is it then, that he can not have the other shotgun options put into the game.

It really only pertains to two options the Reserve Shooter and the Family Business. I feel like although the RS may not be useful, the extra shells in the FB are a huge thing for an Engie.

I understand why the others can't be given to the Heavy/Soldier/Pyro, due to the fact that the WM relies on Metal and the upside for the FJ is revenge kills. But I don't know why it couldn't go the other way.

Thoughts?