it can build bad habits into your muscle memory. hands low, not enough head movement, it may also cause you to miss an opportunity to the noggin in a real fight. i'm against it.
We do a drill like that called "neck on down." Its like body sparring except you are focusing on hand speed, moving your opponent and landing fast combinations. I love it.
its strictly body, you start out strait punches to get inside and once you do throw a fast 3-5 punch combo to the body. The other guy works on defense and footwork. And you take turns. Do it at a fast pace, its fun.
We did some body work at my gym yesterday, we did jabs to the head+ anything to the body... it keeps you honest yet has the same effect