If you were a user that used ePub, would you want the ePub you were reading to be made a well as it can be?
Yes, of course I would, JS, but if I can format my stuff so that the reader can't really tell whether it possibly could be better, but it works for either part of the market, then that's the sensible way to go. I agree, the 90%-10% split is probably a bit wrong, but it is certainly massively in favor of MOBI for the simple reason that Amazon has stuck to its guns for Kindle marketing reasons. In the longer term, who knows? Perhaps they will relent and upgrade the Kindle firmware to allow ePub files because books with illustrations are typically second-rate in MOBI format? That might happen as tablets gain market share over eBook Readers, and users can compare screens.
But, as a novelist, my stuff is 99.9% text.... I don't really care too much!