Generally, I would say your right. A professional or higher class sportsperson turned Coach/Trainer should have a higher level of knowledge/input to someone than somebody who hasn't performed at the highest levels. I listen to the likes of Tarver, B-Hop, RJJ and they are all intelligent, knowledgeable guys who you'd think could relate to a top-level fighter more than someone who hasn't boxed professionally.
On the flipside, I think to be a top coach, you have to apply your knowledge in the right way, otherwise theres no point you been there. Hunter has probably been observing and studying around boxing gyms for years and can probably teach the correct fundamentals, but doesnt have the ability to apply them himself otherwise he'd be world champion and not Ward! Also other non ex-pro trainers, the likes of Adam Booth, Pedro Diaz et al. have sports science and/or psychology qualifications, so can earn the respect of a fighter in a different way than somebody who was an ex-champion in your corner.