I've gone back an forth with this. Currently we have everything in one cluster and have not had an issue however, we do keep our vCenter and database on separated physical serevers. If I had a separate cluster for management, I would virtualize these as well.
Beyond that, I can't think of additional reasons to do so. We're designing a new environment and I am revisiting this. Can anyone offer pros/cons for this? Perhaps with regards to virtual backups, SRM or other tool sets? At face value it seems logical but, I haven't been able to put together anything strong enough to justify it.
