Good point. As ex riders, Melandri and Biaggi's testimony has some value, but we should take their words with a grain of salt, particularly Biaggi's. What are such competitive personalities going to say about a league in which they did not achieve as much as they hoped they would?
Some people are just winners. This is true in all sports. Beyond that, some can repeat success while there are those who put together one winning effort, or several spaced-out victories. Nicky, as much as I like him, is no Rossi. Spies, on the other hand, he seems to have what it takes. Many said he benefited from being on a Yosh bike in the AMA, yet when he went to WSBK he set the series on fire! New team, new bike, new tracks, new competitors, new series, new pressures...none of it mattered.
Then he shined in his rookie year in GP, what's next?
Jumkie, remember the Atlanta Braves in the 90s? With Glavine, Smoltz, Maddux and other stars, they were supposed to be the team of the decade. They only won one World Series and were beaten twice by the Yankees (once in an upset).
What about David Coulthard? He was with Williams and then McLaren when they had championship-winning machines, but how many titles did he win?
Having it all is one thing, getting the job done is another. I know this isn't terribly technical or insightful, but I think it is something we should keep in mind when trying to figure out why some riders succeed while others don't.