[Spiffy]

Yeah... file sizes.

Overall its probably not a HUGE issue with the ginormous memory cards available these days, but I have to think that a format which tends towards a smaller (more efficient?) file size might load faster and be a bit less error-prone.

Don't ePubs tend to be a bit bloated compared to straight zipped HTMLs? At least that seems the pattern from ones I've spat out of Calibre conversions.