err.. first of all mimic only lasts 20 minutes, and the ground raid lasts about 25 minutes , and that's not including the time it takes to warp and wait for aliens to come after lowering cloak. Also the ground raid is pretty easy so you don't need to be in mimic soldier/doc anyway.
I would happily go mimic engi if it makes docs happier about joining the raid
P.S. when are they gonna fix the issues? i thought these changes might happen on this patch :S