[jpalomaki]

Maybe it is easier to keep the maximum amount of virtual machines running on your hardware when you only offering fixed sizes? You can plan your physical hardware so that there is always certain amount of vms on one host and all CPU/mem/disk is allocated to them.

With more flexible allocation of resources the pool would start to fragment. Without local disk the defragmentation process would be fairly easy as you could just restart the vm's in another host, but local disk makes this more difficult (or more annoying for the customer).