The advantage of low cost single-purpose machines. We use our virtual systems to run our general hosting business, MirroredHosting.com. Virtual machines let us build single-purpose machines. For instance, we have some virtuals that do nothing but DNS, others that do e-mail, and others that merely virus- and spam-filter. The advantages of this approach are several:

First, we can lock down a machine tighter and more securely by this means because each is single-purpose rather than multipurpose. Second, we can build up machines far more easily because there are no conflicts between applications to resolve. Finally, if things go wrong, we can rebuild or reboot virtual machines far more rapidly than we can a physical machine -- a kernel panic on a virtual does not halt the underlying hardware.

Our machines are of low enough cost that you, also, can afford to not put all your eggs in one basket, and can instead make setup and maintenance easier for yourself by building single-purpose systems.