Yeah. Didn't Rainey win his titles with his team switching from Michelin to Dunlop then back again or vice-versa (it's a while ago now, my feeble memory doesn't go back that far)? I was going to say that perhaps these days, things are so competitive that riders need every advantage they can get, thus getting prefered tires then sticking to them, but as in Raineys case above, the racing was pretty competitive back then as well.
Your statement's got me thinking about how Bostrom would go as a development rider. If he can't sort out his differences with simple tyres, how is he going to go if Ducati bring out a complete new bike as well (as rumoured) in a year or two, and Bostrom has to try to sort out the problems that will invariably occur with it. Maybe that's another reason to keep Bayliss around as well.