I question why Zell Miller would be invited to speak and not someone like Alan Keyes. Keyes is more conservative than HE is.
In that case I see your point. (See how much better it is to explain rather than assert?)
Yes, Keyes is probably the most conservative candidate the GOP has had for years and they won't embrace him. I think it is the embittered, grudge carrying nature the RNC has against candidates that don't shut their mouths when the grand poohbahs say the race is over, like they did in CA to McClintock.
That or it is just that the RNC worries Keyes will be "too conservative" for primetime. Either way is bad for conservatism I agree.