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the main point being that it isn't up to Rossi to tell them how to cure the problem.
Exactly - just as it wasn't Rossi who told Yamaha or Honda how to cure any problems they may have had. The riders give their impression of what the bike is or isn't doing and of what they want / don't want it to do. With their team they can mess about with set-up but once it goes beyond that it is the engineering and design teams that take over in the search for a solution to a problem. This applies to all riders, not just Rossi, but it is only Rossi who ever gets the accolades for providing meaningful feedback.
I'll tell you why, because he's probably no different from most others in the feedback he gives (Ducati were very impressed with Redding's feedback apparently) The difference is that Rossi is one of the best Motogp riders from the last 12 years. When a bike is working how he wants it, he is good enough to use it to its full potential. Other riders may be just as good, some may be even better, at working with the enginners, but they aren't good enough to ride the end product to it's full potential. Therefore the bike appears a dud. Edwards never got results with the M1 - you have to take into account he was used as a test mule at times, but nonetheless, not in Rossi's league. If Rossi hadn't gone there, whoever they hired instead may not have achieved results either and the M1 would have slipped by unoticed, its potential unfulfilled.
Then Honda. From 2007 - 2010 Pedrosa scored a few wins - again, may have been more but for injuries, but probably no title. Dovi got one win. General opinion was that they couldn't sort a bike, no development direction. But was the bike not good enough, or was it another case of good bike with unfulfilled potential? Then Stoner got on it, 10 wins in one season. So you would have to say that Pedrosa's work with the engineers was positive in producing a competitive bike, Honda just didn't have the rider to fulfill the potential. Had Stoner not gone there Lorenzo would have a second title, Pedrosa would be labelled again as someone who can't give direction to the team.
It's too easy to give credit to one person, usually the one with the highest profile. There is no doubt it is a team effort to hone a bike, but the lions share of the credit belongs to the design and engineering team. Most riders will be capable of telling them how the bike needs to behave, but ultimate responsibility to produce the competitive machinery rests with the engineers.
Too much credit goes to the rider as he is visible, the engineers are not a tangible entity, therefore no-one identifies with them.
