I think there is a consensus that brand-switching is primarily a business decision; that most drummers play on a variety of brands regardless of their career stature or endorsement status. It happens, sometimes by choice (a certain drum sounds better for a certain application) or by chance (festival backline is brand x and there are no available options.
What does bother me, specific to the Russ Miller situation, is that it would appear that he researched/was courted by Mapex for a while, yet he remained aligned with Yamaha right until the announced switch. In this case, what does Russ' "endorsement" (in the sense of "his recommendation") really mean? How can one brand be perfect/essential, and then next week it is not? If I was Yamaha and found out I'd been buying ads to feature this guy in the same quarter that he's getting Mapex test-products shipped to his house, I'd be pissed.
I am not suggesting Mapex's product is inferior, I am sure it is good stuff, I've never played any of their drums, but I do think Russ Miller's opinion on what I should buy is suspect. The clinic/endorser route is a necessary career path these days; the high-end live work is more competitive than before (how do we edge our way in when Vinnie, Gadd and Steve Jordan are willing to tour 9months out of the year?) , and studio work is a dying art. I can only imagine the money RM has sunk into his home studio (yikes!). With that in mind, how you navigate that career path is important, because in the absence of live work or high-profile recordings, how do we assess a given drummer's talent/contribution/authority that makes him a spokesperson for a drum company?
I think those that choose this path should adopt a policy of "going dark" for a reasonable period when they make wholesale shifts in endorsements. There were some hilarious moments in old issues of MD where Cobham and (maybe Carmine) were caught endorsing competing products in the SAME ISSUE by virtue of advertising lead times...