As a GM, I'd be very wary about using arms to get around the MP values for drone weapons. Cyberarm capacity is really permissive while drone capacity (eqv.) is fairly restrictive. Using the former to circumvent the latter gets pretty cheesy. A full-sized arm with a grenade launcher on a fly spy should be one of those "final arbitrator" moments.
Oh, yeah, definitely understand that part of it.
My actual build was going for a Drone Arm on an Aeroquip Dustoff, so I figure the Drone Arm on that would totally have the capacity to put some ... stuff in it. And a fairly large arm on a Dustoff just makes sense so you can touch down, grab a body and shove it inside you, never to be seen again.
EDIT: That said, the main thing I was wondering about was Agility Enhancement. The implanted cyberweapons were just a bonus question after I started along the logical path. After all, if you just have a drone arm, you can totally pick up an SMG or a sword and start blazing or hacking.