A lot of Android fans may be excited about Material Design, sure, but the same was true with Apple fans on iOS 7 - didn't change the fact that a huge number of our users hated it. And while it would certainly be easier to give everybody the same UI, I'm a little worried about changing things again on people so soon after last summer's redesign - a lot of our users are not especially computer-savvy and already had an uncomfortable couple of weeks getting used to that new design.
What we might end up doing is adopting AppCompat now (so we only have to design around one theme) but keep it somewhat Holo-ish in its details - gray (but Material) toolbars, e.g. - and then introduce other changes more gradually: tinted toolbars in one update, CardLayout in another, and so on. A lot of them can be done as easily-rolled-back options, too - we're pretty thorough in our use of themes as of Pleco 3 and that makes app-wide changes relatively easy to apply - so if we pull the trigger on blue toolbars and get a bunch of angry emails about how distracting that giant block of color at the top of the screen is (we have in fact had quite a few emails of that sort over the past year) we won't be so committed to it that we can't easily add a checkbox to go back to gray.
But basically, having ridden out a month or so of angry emails about v3 on iOS and then another month or so of them about v3 on Android, I'm inclined to take a more careful / conservative approach to UI changes going forward.