Why rent more servers when it's so much easier to spin them up as you need them in AWS and tear them down (and no longer pay for them) when you don't. If you're expecting unknown change in scale, it's so much easier to be able to just spin up servers to keep up than anything else.

> To handle peaks, it is both simpler and cheaper to keep enough capacity just idling around than spinning up and down Amazon instances.

At a growing website, you have no idea how much "enough" is. Why try to estimate caps when you don't have to?