I find them extremely valuable actually. The rocket fist was a guaranteed 24 damage fully upgraded and the flamethrower took care of chyrsasslids like butter. I've only ever used heavies as Mech soldiers however, since they basically replace them and are more versatile

But thats actual game.

RP-wise they will have either two roles: support or tank. I mean one of my favorite part of the mechs was fighting muton beserkers and beating the shit out of them in melee combat