BARO sensor input vs. MAP vs. altitude

Noticed newer GM MEFI controllers have capacity for separate BARO sensor for "altitude compensation", according to GM MEFI literature. Doesn't the MAP adjust for lower air density with altitude as there is less MAP with increasing altitude? How much better would a separate BARO sensor be in speed-density? ECM's normally register BARO, or ambient pressure, at start up, then by either MAP or MAF once started. Would a separate BARO constantly sensing changes in ambient pressure be that much more accurate? Anyone have any experience with this issue?