Plus, many companies have very bursty workloads - we can (and often do) scale from 100 servers up to 1000+ servers several times a day. Without AWS we'd be payi

Plus, many companies have very bursty workloads - we can (and often do) scale from 100 servers up to 1000+ servers several times a day. Without AWS we'd be paying for 1000 servers 24x7, now we only pay for them several hours a day. We've run the numbers, and we can't host it ourselves anywhere close to what we pay Amazon if we want the same amount of redundancy that AWS gives us "for free" -- we'd have to duplicate most of our infrastructure across 4 separate datacenters to get the kind of multi-AZ, Multi-region redundancy we have now.