I wasn't trying to claim that it couldn't happen, or that you didn't have it happen, only that I believe that it shouldn't happen. To be fair, I've seen similar problems back in the ESX 3.5 days and earlier. There were some known issues with snapshot removal that could cause this. But since 3.5 U2 (I think U2, I guess it might have been U3) snapshot removal was overhauled completely and now uses helper snapshots in a loop until the final snapshot is small, and thus the "stun" time should be short. Veeam 4.x also has "safe snapshot removal" that let's you get similar behavior from older versions of ESX.
I guess what I'm saying is, if you're seeing this with current VMware versions, well, that still seems like a problem, perhaps something unique in your environment (slow storage, the virtual RDM's you mention -- we use VMDK's, etc). In other words, I'm fully buying that it can happen, but if it were happening to me, I don't think I'd let VMware off the hook with the "don't preform a backup of a busy VM" excuse. What if I were using snapshots for other purposes?