It seems evident that unenforcable rules will vanish over time. There
was a time when rules in certain competitions prohibited the use of
any other frequency than the competition frequency, to guarantee
individual performance instead of team flying. Relatively easy to
control when crystal radios were the norm. Difficult to enforce once
small portable720 channel radios became available. Completely
impossible to enforce once cell phones became widespread. I don't
think there still is one single competition using this rule. And team
flying is now promoted in international competition. The same happened
with GPS. First banned, now mandatory.

In this case, I think it was most unwise to develop a AH-function in
the XCSoar program, if competition flying was envisioned at the start.
The AH and every other system permitting cloud flying, includung some
types of compasses (Bohli etc.), have been banned from soaring
competition for a long time. But given the numerous stand-alone AH
apps for every type of PDA/PNA/tablet/smartphone, I don't see how this
rule can still be enforced today.

By the way: if you favour an "honor" system to control compliance of
the AH rule, you could just as well believe a pilot when he tells you
he won't go / hasn't been going IMC. The same people (if any) will try
to cheat...