Yeah, my Ofna Hyper8 buggy uses this arrangement (see Conversions link in sig). It does help balance the weight, but room is at a premium in buggies and with dual packs, not much room is left to mount the other stuff unless you get creative. The side mounts are probably easier to make (and therefore cheaper). Also, like MTBikerTim said, it raises the CG.
I made my own mount for my S3 over the dif and flex is a big issue. The S3 had a channel down the center so I added a convex rubber pad under the end of the motor to absorb some of the impact, but the pinion still showed wear. I tried to talk Mike into making me a mount for a Team Magic buggy and after he desribed how much of a pain and the downfalls, I went with his RC8 mount.
the pic was taken with a cel so not the best. I had 2 of these buggies, and the nitro I used the extended chassis. the stiffness of the S3 chassis with the channel and the centre line drivetrain made this my option and it balanced good. I sold/traded them to a buddy to get another brushless buggy in the group to race and started the next one. yes it has pink sorta straps, so is that ugly carbon though so it matched LOL