I don't think that's a universally agreed truth. There's a school of thought that the stirations from machine tuners may increase grip.
I'm not sure if I necessarily believe that, but I don't feel the need to polish the skis further after completing my passes with the Razor Tune (other than to tidy up tips and tails in some cases). I don't notice any adverse ski performance characteristics from making my final pass with the Razor Tune when compared to a hand tune.
Wade, my reason to recommend using a diamond stone after a disc grinder is not because it increases performance of the ski, but because anecdotally it may reduce laceration injuries. Looking at an edge tuned with a stone grinder it is clear that there are micro-serrations aligned perpendicular to the length of the edge. Like a very fine steak knife. Those serrations do, in fact, seem to do a wicked job of gripping ice, but they also seem to make the edge more capable of cutting through fabric and flesh. I have no real evidence to back this up, but I've heard this speculated on numerous times at Eastern Division USSA meetings and at the annual Stop the Bleed training. Certainly, there is a correlation between the rise in use of disc grinders and the number of laceration injuries, but I appreciate that correlation does not equal causation, and even more so, that the serrations may be much less of a factor in injuries than the fact that disc grinders simply do their job of sharpening very well. Maybe I'm deluding myself in thinking that removing the serrations reduces risk.
At the end of the day, the one very clear thing is that wearing cut-proofs significantly reduces risk. A couple of seasons ago, my daughter went into some A-net and her ski cut her suit as cleanly as a razor blade would, but her cut proof base layer completely prevented a laceration.
