"They're not class legal in most catamaran classes. "
You mean like, one-(manufacturer)design classes ??

We`ve seen this before, mylar sails are still outlawed in many class rules that were written in the `70`s, despite them having become cheaper and more durable, hold their shape longer etc. I guess the reason might be that if, in your particular boat`s class rules, it states that you have to buy your sails from one supplier, normally after being marked up by the "license holder" (ie manufacturer), it`s not in their best interest that the buying public have sails that last indefinitely and have a long racing life.
That`s the cost of "keeping the costs down"