See the title. I actually made this change many years ago in my mod, because it's fairly obvious to me that a TMD weapon is a much more obvious choice for a combat engineer than a crappy gun that can hardly kill T1 tanks. Let's discuss.
First, the main role of the Sparky is to build PD, both T1 and T2, near the front line. What naturally counters the PD? A TML or MMLs. Obviously the Sparkies should have TMD to pre-empt that counter. The PD will kill nearby ground units, so the sparkies don't need their own guns for that.
Second, to piggy-back off Deribus's call for a fatboy buff, it's obvious that a mobile TMD escort could greatly improve the fatboy's survivability against TML. And it would be nice to give UEF something unique. And have I mentioned the current pea shooter is basically useless?