While the redundancy would be nice, I can't see it being worth the effort/money for anything but the most critical of systems (which should probably be self-hosted in their entirety to begin with). If the internet (or some major swath thereof) is down, that's going to cripple a lot of businesses these days regardless.

I just don't see it being worth the extra effort of self-hosting a redundant copy of the system, and then keeping that system up to date with the latest software for security patches / features / bugfixes. Especially when the cloud version has n x 9's of uptime.