VMs tend to occupy a lot of memory, but they are normally also the official denizens of a server. So if memory gets tight we rather have the oom killer kill the new kid on the block (some process which has suddenly started using more ram) instead of the regular VM crowd.
Obviously bad things will happen if the oom killer is not able to free enough memory to make your machine happy again, but with this you at least have some control over who gets killed.