If you actually crunch the numbers, you'll see that the likelihood of multiple node simultaneous failure of a larger cluster is actually often as high, or higher than the likelihood of a single node failure on a non-distributed application.
Despite running redundant distributed set-ups, services today really don't have particularly spectacular availability. Google, Facebook, Twitter, Github; all go down or brown out frequently.
If you actually crunch the numbers, you'll see that the likelihood of multiple node simultaneous failure of a larger cluster is actually often as high, or higher than the likelihood of a single node failure on a non-distributed application.
Despite running redundant distributed set-ups, services today really don't have particularly spectacular availability. Google, Facebook, Twitter, Github; all go down or brown out frequently.