I believe there was a thread several years back posted by Hoss on various upgrades for the G0704, One of the mods was replacing the OEM spindle bearings with high speed bearings.i was under the impression that we are talking a milling machine here. If so there are plenty of times where a high speed spindle is the right answer. This is even more so if the Mill is to be CNC'ed. Even manually running ver small cutters and burrs on a Bridgeport can often benefit from higher spindle RPMs.
High speed spindles are quite common - I have seen some on commercial multi-axis machining centers that can run in excess of 30,000 rpm. Precision balancing of the tooling is a must with high speed spindles.