Yeah, but that objection applies to basically any weapon put in the heavy slot. (Unless Graviton Thruster Lances go right through really thick cover, I suppose.)Sweepers do seem like they'd pair well with mortars, but I don't know that putting them on the same vehicle is the right way to pair them.
The heavy probably only gets fired versus chumps or as part of running away, and a Sweeper sounds like it might make running away a lot easier.