The same advantage you describe exists in other "above kernel" virtualization solutions, such as Solaris Zones. If you use a sparse root model then many aspects of OS configuration are shared between zones. This can simplify maintenance if all apps are happy with the same config, but if the apps have different needs then each needs to be given its own environment (which, in the case of zones, is called "whole root").
If one looks at the spectrum of options, in general the higher up the application stack you consolidate the more you share (and, ususally, the higher the ROI) but the less flexibility you have. Consolidating databases or app server content gets rid of physical boxes, OS images, etc. but requires more diligence when planning and managing. Below-kernel VMs only get rid of hardware, as you still have an OS for each environment (and all the accompanying maintenance). Their advantage is that it is relatively transparent to the application.
Another topic that I find very interesting is the management of change freezes in virtual environments with motioning enabled. This is an entirely new frontier, and if you freeze everything that you probably should then you may actually prevent the cluster from balancing itself. Also, performing maintenance on OS images in VM environments where memory sharing is present can potentially affect other VMs by reducing the amount of sharing. If this creates memory demand then a frozen app may actually start paging...
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