Cost is a big one. Having shared storage is almost a must with virtualization if you want to take advantage of some of the more advanced features, such as high-availability [HA] and vMotion, that require shared storage. The cost of implementing a typical Fibre Channel solution for shared storage is typically pretty high. A NAS solution, on the other hand, can greatly reduce the amount of the expense of implementing a shared-storage solution because NAS uses common NICs instead of expensive Fibre Channel adapters. [NAS also] uses traditional network components instead of expensive Fibre Channel switches and cables.
Cheers, Yours Udin 