[watty]

I'm not sure I follow. Obviously if my host goes down my entire site will go down - this is not the point. The point is that when my host is up, what % of the time will my site still be down? Well if you use a CDN that % is greater than zero. If you use two CDN's the percentage is compounded.

[k3n]

And that's why you have a fallback, to fully mitigate any CDN downtime (how little that it may be). It's like a spare tire in your trunk.