Out of curiosity - Denny, what yeast strains have you encountered increased ester production when overpitching?
My experience, as a lazy and cheap brewer who tends to use yeast cakes over and over, is that of the others who have posted here. That is, English and weissbier strains benefit from underpitching to achieve the ester character in the final product. Having known you on various forums for many years, Denny, I am fairly certain that your experience on overpitching is not drawing on British bitters or hefewieizens.
In fact, this subject seems to come up with some regularity, and as far as I am aware of, only you and Dr. Cone have had excessive ester production resulting from overpitching. Which makes me wonder if it is 1450 thing. Or maybe an APA/IPA thing, the hop debris in the trub is causing some effect.
My experience: besides the above, overpitching results in greater attenuation, especially with ale yeast. Lager yeasts, I think you can pitch and pitch again on top of yeast cakes with no adverse affects, as long as you keep it clean. I have gone up to 7 times with pilsner yeasts with no noticeable off-tastes.