We have been looking at a number of services in our own hosting environment that are “spikey”, things like image manipulation, video encoding, marketing email blasts, etc. By offloading the spikes from our origin servers, we get better efficiency out of those machines — I.e. they are handling consistent loads throughout the year. We push the spikes onto S3 and EC2. Amazon’s clients have a wide variety of needs, which help to even out their loads. I might be driving high load today at EC2, but tomorrow my traffic might be sleepy during someone else’s peak.