They're very useful if you're testing a concept, need agile scaling of computational power, or are just starting a service and don't want to / can't invest the

They're very useful if you're testing a concept, need agile scaling of computational power, or are just starting a service and don't want to / can't invest the capital in dedicated hardware. I agree with you on your last point though, making your service entirely dependent on these services makes you little more than a franchise and is a potential vulnerability if you ever compete with any important existing service. It probably isn't a good idea for a mature or rapidly maturing business to rely heavily on these services.