Wow! What a great thread that is hidden under this crappy title. I had not considered looking at it until now and am glad I did.
First to the OP's question, I agree that the proper stir plate rpm is that which keeps the yeast in suspension. There is no need or advantage to high rpm and a frothy vortex.
Mark, you have incredible knowledge shared here. I agree that a stir plate is not ideal, but I don't think that its terrible. One thing that is true, is that a stir plate alone is not sufficient to keep a starter aerobic when using typical narrow-ended vessels. There is little chance that there will be enough exchange with the normal atmosphere to transfer enough oxygen. That is why I flood the headspace of my starter flask with ambient air that is filtered through a 0.45 micron filter. That way, I know there IS enough oxygen present in the headspace to transfer into the gently-stirred wort below. Mark, is your problem with stir plates the fact that they aren't keeping the wort aerobic...or some other reason?
According to (Verstrepen KJ, et al. “Yeast flocculation: What Brewers Should Know.” Applied Microbiology and Biotechnology, Vol 61, pp 197-205, 2003.), flocculation is generally inhibited during active fermentation by the presence of mannose, maltose, glucose, and sucrose in the wort. So that means that if the yeast tend to quickly settle when you stop stirring, the wort is exhausted. With the information in this thread from Mark, I now know that I should get that starter into the fridge to settle for decanting. I have probably been continuing the stirring for too long. I'll revise my procedures.
Mark, I read your contention that temperature control is not that important. But I'm concerned that your example yeast species may not be a true case in point. While it apparently works for that relatively clean fermenting yeast, I'm curious if that result can be applied across the entire yeast spectrum? I anticipate that it can't. Can you expand on that?