Apologies, It's been a long week. I thought you were talking about OD in general, not just F18.
Personally, I see F18 and A-class as one-design for fleet racing. I know, that's not the correct definition, but if you're racing boat-for-boat without handicapping, that's close enough for me.
Yes, the rules of these classes evolve, but that's the price to pay if you want to race those boats. I don't think anyone goes into it not knowing that (or, they shouldn't unless they've been living under a rock). SMOD is a different animal, which I personally prefer, but everyone has a right to their choice.
BUT, that's not to say I agree that Hobie class regattas should be open to non-Hobie F18s. That has more to do with the support and purpose of the HCA and HCC.
Mike