Are you sure none of the tempered area is left? Give it a good soak in vinegar and the temper line should show up... if there's still some tempered steel left, then yes - it's usable given a good reprofiling and sharpening. If not, then it's probably a goner.
Even if there is some tempered steel left, you have to assess the profile of the head.. if enough of the bit has been worn or grinded away, the cutting edge/profile that's left will probably be very thick and not bite into wood nearly as well as it would have in its original state.