All opinions valid?
You know, that's a good question. I don't debate much or get into very many arguments over things that require an opinion. An opinion is like when two girls go clothes shopping and ask each other how they look in their new outfits. Neither one is definitively right (at least at a man's point of view), but, perhaps, to the girls, they certainly know that they're right.
An opinion isn't generally considered a fact and isn't authoritative. Opinions are found a lot in politics and politics isn't really a science of accuracy at any length and doesn't operate on truth and data (except at the polls), but even then, votes get periwinkled a lot, so really, where does truth and fact really exist in politics?
Opinions are often used as underhanded deviations and masked insults or tactical conversation devices to steer the subject or arm-wrestle with it. I don't really know how to answer this without saying that opinions are for girls.