[Freebytes]

When infrastructure costs are your largest business expense, then having two separate providers for a duplication of services is not feasible. You must also then have methods by which to determine when the service is unavailable and seamlessly fail over to the service that is actively running.

In cases where data integrity is important, this could also result in splits where data gets left behind. It is often better to fail than to have situations where the data does not match.

[dnadler]

Well, to address the costs issue, you could run hot-hot and have half of your resources on each provider, and in the event of failure, scale the functioning one up to meet demand. That's obviously a decent amount of effort to set up, though.