Maybe there should be a rule that says you are only allowed to place 1 post for every hour you sail!

Just to be clear, the F18 rules say nothing about changing crew during a regatta: http://www.f18-international.org/F18_2007_Class_Rules.pdf
Its the NoR/SI that should cover these issues, I think changing crew is fine as long as there is a valid reason and it is approved by the race organizers.
IMHO if you dont it might reak a little of opportunism when you switch between crews or even go uni during a regatta to suit conditions.
Its not fair to the crews either, because if there is a lowwind day and the skipper decides to sail uni.

What might be a fun idea for an F16-only event though is that you are allowed 1 change during the course of a regatta.

(Timbo: maybe you should apologize for calling other sailors Nazis, that is totally ridiculous.)