I think with current sail technology in regards to the J/30 mainsail there is almost no advantage with exotic materials. So one could argue that we should ease the restriction. However what happens if next year there is a breakthrough on mainsail technology but they are 10k a piece and anyone that has it will smoke people that do not. We have ruined the one design class racing. The spirit of the rule on mainsail cloth restrictions is - no arms races - and currently a well designed main of any material is going to make the boat go well so currently I do not think we have an arms race in the class and that is a very good thing.
Numerous other very large classes restrict to Dacron because of this very reason (J/24, J/105, Etchells, Sonars, etc.). It protects the class from technology out pacing the rules in an unfair way.
That being said I really do not have a problem with protecting the "spirit of the rule" and allowing other material as long as it protects from the "killer sail" showing up right before the North Americans.
It is a reality that most of us race PHRF and the occasional OD and if slight modifications to the rules increased OD participation then I think that is a postitive thing for the class.