I'm not sure that under most situations, Marriott's 'miles' earning option is ever worth it. You can still transfer points into miles using a transfer award instead of a TP award. And doing that will, in the end, give you at least the same general miles as if you directly earned them (or most likely better.) I think aaupgrade or maybe someone else did a comparison of miles verus points earnings. And it almost was never worth earning miles directly.

Also, I do believe that you could have problems with some of Marriott's bonus promos as those may require you to earn points to get the bonus. So if you change to miles, those stays may end up not getting you any credits for those bonuses. And if you forget to change back, or heaven forbid Marriott doesn't register your changing back when they should have, you could end up not earning what you thought you could.