I'm going to attempt to clarify the opinion problem from the opening post.

Everyone has opinions, some of which may not be rationally founded.

No problem with that, whether rationally founded or not. No "onus" is created on the opinion-holder at that stage. Or better expressed, I think, there is no burden or proof.

Many, if not most, opinion-holders will from time to time state their opinion(s). No problem with that, as far as it goes, and no onus or burden or proof follows from it.

However, when a speaker or writer expressly or impliedly calls on other individuals or society in general to believe and/or act on an opinion he expresses or endorses, there is a burden placed on him to at least attempt to substantiate the opinion. In the absence of substantiation, there are results that follow. Some of them are:

1. Hearers/readers are likely to discount the opinion out of hand.
2. Hearers/readers are at least tempted to reject other opinions put forward by what I'll call the initial opinion-expresser, even if an explanation is attempted in that case.
3. Such blanket rejection might be applied to others who echo or support the unsubstantiated opinion, unless they supply the missing rational basis or at least attempt to do so.

Dave Oesterreich