In some cases it appears that experts who are less certain about their opinions more persuasive and compelling.
Why? This is counterintuitive. Basically, what they think is going on is an expert that expresses uncertainty are surprising. We expect experts to know but when they surprise us we feel something is amiss and we pay more attention. If the message is compelling and we pay more attention we're more apt to be convinced. This can backfire. If experts admit uncertainty with a weak argument it becomes less compelling.
What about legal implications? Courts are more convinced when testimony is given from experts who are certain. However, eye witnesses are often giving testimony in a context where there is an objective truth.
Maybe this only works in subjective domains.