even our robodrills at work use a break(or clamping mechanism) to hold the 4th/5th axis in place...
out okuma lathes that are servo driven can obtain excellent position but lack holding power(not really needed on our setup as its used for turning only) I can put an allen wrench into an M8 bolt on the spindle and easily pull it an inch as it fights back, and I have also faulted the servos out trying to break bolts loose.
you would have to have some sort of break for sure if you were doing any heavy duty cutting. I think for hobby purpose swapping out a good DC motor and a stepper is not such a bad option as long as your mounting is clever.
http://www.g0704.blogspot.com/