Two other thoughts;

1) I might agree that for a brand new pilot that AGL may be easier to
learn, but that doesn't make it right, nor make sense in the long
run. Primacy, primacy, primacy.

2) Once you convert, go all the way - don't make it optional - half
measures can cause problems. Here is a scenario that happened to me
while we were in the "optional" mode. I flew a club ship and used MSL
(888ft). I land and another pilot jumps in and thinks, "Heck, the air
pressure must have changed so all I need to do is tweak the big hand
back to zero". So he reset the altimeter to AGL by turning the hands
CLOCKWISE. See where this is going? What he did was move the
altimeter setting by +112ft, not -888ft. So now the altimeter is at
1,000AGL, not zero AGL, but he doesn't notice. The other pilot gets
off tow at 2,000ft AGL (thinking he is at 3,000ft AGL). The tow pilot
notices the other pilot getting off 1,000ft early but doesn't think
much about it (until later). At some point the glider pilot thinks
something like, "Wow, things sure look big", promptly lands out and
dings the glider. A CFIG comes up to me afterwards and says, "You
should have reset the altimeter back to 0 AGL after your flight."
Huh? My comment was, "Who exactly is in command of the glider? Me
standing on the ground or the guy in the cockpit?" End of
conversation.

Name withheld for fear of bringing up bad memories in others....