Ubuntu aims for ten-second boot time with 10.04 (ars Technica)

If you are creating and destroying virtual machines in response to load, it is nice if they start up fast since the time spent starting the VM is waste and costs money.

Things like hibernation or suspending the VM are not particularly interesting for this model either. Each VM will usually be a clone of a smaller set of machine images, so the state of individual VMs is not that interesting. Further more, if you did suspend/hibernate the VM you'd need to store that image somewhere and that would also cost money.

These benefits are most obvious when using leased resources like Amazon EC2, but can also be relevant for private clouds: the time spent starting a VM is time that can't be used by another VM.