The difference may be big enough to put a ground crew on the wrong side of an obstacle.
The real issue seems to be the conversion between UTM and Lat/Lon. Not having a common standard is a bad thing in stress situations. This introduces an additional opportunity for making mistakes. The decission is not easy as the ground crew can calculate distances from UTM more easily and experienced members may even know the location from the coordinates. Air crews may prefer the Lat/Lon format.
I´d prefer the Lat/Lon format as it is more continous because there are no zones. With GPSrs being common and mostly reliable the calculations should no be too hard to do. I´d wait until the maps have to be replaced anyway. I guess that it would be expensiv to throw away maps in good condition just for the sake of standardizing.