There is a place for virtualization. If you can justify the overhead for hands free network management. If your systems really have enough power they can run hadoop and something else. If you need to run multiple truly isolated versions of hadoop (selling some type of hadoop grid services?)
If you are working on the type of application that requires a system like hadoop, the implication is that you have a large network of physical hardware and you need all the power you can get. It is counter intuitive to start splitting up hosts into virtual machines, however every application is different and it may make sense.
