My employer is running two web servers at two different hosting providers (1&1, Strato). Both webserver IPs are A records for the www hostname. The domains are managed by a third ISP (InternetX). 1&1 currently has a major issue with some of their servers and as a result our webserver is not reachable for more than 30 minutes now. So I decided to remove that one from the list of A records (TTL of the zone is just five minutes).
Now guess what: One of the four authoritative name servers of our InternetX managed domain is hosted at 1&1. As a result of their outage the InternetX AutoDNS system refuses to accept changes of zone data. WTF!
And that is exactly the reason why I prefer to run things on my own!
Note: Carefully audit you ISP infrastructure before signing up the contract.