I would have thought that a stepper can actually spin pretty fast if programmed correctly though perhaps im wrong there.
*Please anyone feel free to correct me if im wrong here*
I believe, that if you are running Mach at 35,000 mhz kernel speed, that implies that it can make 35,000 pulses per second, meaning 35,000 steps per second, which at a full step would be 175 rotations per second which is 10,500 RPM. Now, the caveat is, that is a 1:1 ratio which obviously wont work, and 10,000 is probably beyond the rated RPM for a stepper. If you throw an 8:1 ration on there, you're looking at 1312 RPM (at the 4th axis) with the stepper running 10,500 RPM. Also, as Simpson mentioned, the faster a stepper turns, the more torque/holding power it loses. So basically, you'd be at the point where it would be pointless not to do a Servo if you are planning to do high speed operations.
That all being said, I still went with a stepper because of cost, ease of setup, and I didnt need to do lathe work on my mill. Its all about what your intended use is for the machine.