Even for sketchpad, conference etc. type purposes I don't know why built-in mics would be worth the cost over using a little plug in electret like the one supplied, or an upgrade or different pick-up pattern.
What I would want is a quality preamp (w/ XLRs and phantom) + A/D converter, a la an updated Deneke, in a TIDY companion sized case for a front end. Would also allow the microtrack to run 8 hours instead of 3 by not using its internal 48v.