Edit: ninja'd. Yeh, Fluff over efficiency.
Act like Assault Tank, hit like a Assault Mech. ^-^
Why not just build a regular Assault Mech, that acts like an Assault Mech and hits like an Assault Mech? ???
I'm afraid the benefits of vehicle-ness elude me here. Then again, if tptb decide to create one, I probably wouldn't complain.
I'll probably withhold speculation, though. As seen with the Notos, the flexibility of quadvees really comes to the front when they notably gain, like with wheels and the correspondingly high roadspeed.
Heavier designs spend that much more total tonnage on conversion and the secondary drive train, I doubt it'd make much sense to try. Which probably means someone will. ;D