Necessary, or desirable?
In my own case, I've used VMWare's Fusion while my partner used Parallels. In her case, she had no issues since she only used Windows for specific, non-intensive applications.
However, as a photographer and photographic consultant, I needed to use applications such as ProShow Producer and others to generate DVD slideshows using the features and control these apps provided--apps far less expensive than Final Cut Studio, for instance. In one particular case, the disadvantage of working in a virtual machine became patently noticeable because I was timing the transitions down to the millisecond and Fusion consistently slipped timing to where the transitions went as far as fifteen seconds out of synch from the audio. In my business, this is worse than unacceptable--it's unprofessional.
