The variable workload has been a little less important with reserved instances. They used to have a "light/heavy utilization" pricing structure but now it's basically cheaper to reserve the instance than it is to scale it up and down, unless you need the extra capacity less than 60% of the time.
I think you misunderstand me. Doing 100 servers for 16 hours and scaling down to 30 servers for 8 hours a day is more expensive than running 100 reserved instances for 24 hours a day. (100 reserved instances == average of 60 on demand instances).
