Suppose you have an isolated application that's out there "on the edge," it is working and doing its job in a satisfactory manner, demanding few upgrades and minimal maintenance. In a case like this, there is probably no need to impose storage virtualization. But aside from these relatively rare "fringe" applications, there is really no reason to avoid virtualization.
However, readers should be cautious of any storage virtualization product that demands 100% control of the storage environment. You ideally want to apply virtualization in a careful, measured fashion. If you see things shaping up as an all-or-nothing scenario, that should flag a particular concern -- far more than worrying about applications that should not be virtualized. Slow, careful integration will help to keep provisioning under control and avoid confusion in the IT organization. Have a set of guidelines for IT folks that will help ensure consistency in the way that various applications are brought into the storage virtualization environment.
Check out the entire Storage Virtualization FAQ guide.