You can get that flexibility without putting your base load on EC2: Use EC2 (or your "normal" hosts cloud services) to spin up extra servers when needed, and host your base load elsewhere. Extra bonus: It allows you to save even more money by flying closer to the wire with your long-term provisioned servers.

These days there are so many hosts that offer combination of colo + dedicated servers + cloud services that you can seamlessly do both even if you don't want to deal with more than one provider.