It's bugger all to do with reliability. It's two answers to the same problem.

1. If you're being constantly hit with DDoS, why not distribute the service?
2. Given the choice between building a massively dispersed infrastructure from scratch and the huge cost and time this entails to get right, when you have a company you can rent the infrastructure off, why not just do it?

Makes perfect business sense to me. And at the end of the day, MS is a business.