Andersp: but to me its still more rude than to resign a finished game :)

But that really is irrelevant. It's not about what you'd find rude, it's about what an opponent who wishes to continue play would think.

i (and most players here) know the real reason why you dont accept even a friendly invitation

Go on then, spell it out. Make sure you properly include the terms "in protest", "rating system" and "really sucks". And you can get bonus points if you add another sentence or two incorporating "DailyGammon" plus some choice ideas from my profile.