As far as I am aware J-Rose was never "playing Mizuno". He did play MIzuno Blades whilst being a non-contracted player. He was with Taylormade for a long time and then signed for Honma, which didn't quite work out for either party.
After seeing Justin on tv a couple days ago, I believe i was thinking of Luke Donald using Mizunos, not Justin Rose. (Those English golfers all look alike to me) ;-)
My mistake...
Secondary to that, the idea of a club change at all must be scary to someone who actually makes their living with a weird bunch of bent sticks in their hands.
At the few professional tournaments I've ever been to, it was very obvious a large presence on the range was equipment reps trying to get pros to try some new club from their brand. I can't imagine what it takes to convince a professional to drop what's been working and change to something else.