I received a strong opinion statement in an email from an expert in the field who later ended up making enough suggestions that I'm including him as a co-author. Coming from me, or from the paper's general authorial voice, the statement would stick out as an unsubstantiated claim, and his papers don't tend to include opinion statements. This would also be the only place in the paper where I'm quoting an expert. Should I just give it up?
To clarify, I'm not even wanting to include it to bolster the paper's argument, in fact it works against the paper's argument. But the fact that an expert has this opinion is a significant piece of information that readers might want to have.