Double-edge sword. If you have a small environment, you can host alll your virtual servers on a single physical server. However to provide high availability, you should add an additional host. With this in mind, you are provided all your virtual servers high availability function as compared to utilizing either Microsoft clustering per supported application. Not all your virtual servers would normally support a MIcrosoft or 3rd party high availability application.
In one environment, I was able to reduce 40 WWW physical servers to 2 physical servers hosting all 40 WWW web servers. Consider the reduction of the power, cooling, and of course space.