Mark James Boyd wrote:

If so, and this were reliable, there would be no need for
a power-hungry mode C transponder, one could use a GPS based
transponder and the ATC computer could simply spit out
altitude. This isn't so farfetched since that same computer
already compensates for pressure differences.

etc.

You're completely missing the point. We're talking gliders here, and
glider flights usually are uncontrolled. A pressure alitmeter works
without controller and without electricity, is fairly robust and
provides the pilot with everything he needs for airspace issues. If the
pilot is too dumb to compensate for wheather changes and flies into
terrain because he believes his altimeter more than the outside view, so
be it.

Stefan