You can get VM portability and some other advantages of virtualizing Exchange (as with any app). As a con, if you have a well loaded, well running Exchange server, those pros may not outweigh any loss in performance from going to virtual.
It is though important to create some rules arroung virtualization of Exchange. Make the servers movable by setting a max on amount of mailboxes or mailstore size. For example a max of 1000 mailboxes per virtual machine with a max of 100MB mailstore per mailbox will give you more flexibility in moving from one to another physical server. Other rule can be that a virtual machine with Exchange may not exceed for example a thenth of the physical servers capacity.