Now, it seems natural that the next big debate in Cloud computing architecture is going to be around elasticity vs plasticity.
Is your Cloud computing infrastructure able to scale up or down based on demand? or Do you have to plan for peak demand and deploy enough hardware to make sure you have enough capacity to address your demand and SLAs?
If it's the latter, then it's like having a hotel keeping all the lights in every room on just in case some one checks in. How about turning the lights on only when you need them?! The amount of resources (aka moolah) wasted on running infrastructure constantly for peak demand can potentially negate any cost savings achieved through multi-tenancy.
The only arguments that seem to make any sense for plastic computing are with regards to SLAs and the latency involved in bringing on new hardware/software to support increases in demand. To that, I say...invest in some operational analytics. I'm sure if you can track your usage, you can see patterns of use and plan for them. For example, if you're running a sales or finance application, it seems natural that end of month/quarter would be the busiest times and beginning through mid-month, things are not so busy. Turn off the lights when you don't need them.