Stoner won on a Ducati which was unrideable. His feedback then didn't make the bike any better. Why would he be of any use now?
There is a common misconception, widely held, that the Ducati got worse because of Casey Stoner's input, and that therefore he is a very poor development rider. While the Ducati did get worse during the period in which Stoner was at Ducati, that had less to do with Stoner, and more to do with the way the racing department worked at Ducati.
Two examples of Stoner's sensitivity and feedback. Cristian Gabarrini once told my friend and fellow journalist Thomas Baujard of Moto Journal that Stoner came into the pits during a test at Qatar and told him that there was something wrong with the engine. Gabarrini looked at the data, and he and the Ducati engineers checked every aspect of the bike and the engine. They could find nothing. So Gabarrini asked Stoner to go out again, and the Australian reluctantly agreed. Halfway round the track on his out lap, the Ducati engine let go, leaving Stoner stranded. It was yet another confirmation for Gabarrini that he needed to trust Stoner's feelings over the data.
The other example came at this test at Sepang. When asked about how the Michelin tires felt, he gave an explanation for why so many riders had crashed in the afternoon of that day. "There's a little point after probably 45°, that it goes down just a little bit more, that it doesn't seem to match with the rear with some of the profiles that we've tested. And that gives everybody a little bit a nervous feeling, and essentially why people are struggling into Turn 5, a big fast open corner, going in, when the bike goes light, it doesn't like that feeling, and it gets the bike a little nervous, and I think that's when the front wants to break away. Everybody has been having a very similar crash there." The crashes were happening either on the way into the corner, or on the way out, both points where the rider is transitioning across that sensitive area. Other riders will just tell you, "it was a strange crash."
The ultimate proof of the error of putting the Ducati's problems down to Casey Stoner came when Valentino Rossi took his place at the Italian factory. Rossi's development credentials are beyond question: his feedback is exceptional ("like a datalogger" crew chief Silvano Galbusera told me), and he has a proven track record of pointing engineers in the right development direction. Ducati listened to Rossi just as much as they had to Stoner, which is to say not very much. Sure, they made lots of changes, but not the changes which were needed.
Ducati's fortunes were reversed with a change in senior management. Since Gigi Dall'Igna took the place vacated by Filippo Preziosi a year earlier, the factory has gone from strength to strength. Dall'Igna has instituted the organizational changes needed to turn the Ducati from also ran to genuine contender. Dall'Igna listens to his riders, and acts on their input. That's the difference.
Riders don't develop bikes, engineers do. But engineers do a better job at developing bikes when they listen to what their riders have to say. Casey Stoner is definitely a rider worth listening to.