[vidarh]

This is the key. And worth pointing out that the moment you're set up to use cloud services for spikes, the cost of using dedicated services for your base load drops:

You can afford to let the servers handling your base load get much closer to capacity when you know you can scale up near instantly instead of having to provision new servers.

This is the biggest reason for me to run services that are prepared to run on public clouds, though it's very rare I've ever needed to make use of it - the kind of spikes that are severe enough and long lasting enough to be worth provisioning cloud instances for tends to be very rare for most people.