Comment by FrankPetrilli

"Rarity" is a distinction without merit in this particular case; the important thing to note is that (most) clouds don't guarantee _any_ availability of a single zone. A system which stashes all of its infrastructure in one zone only is expected to be impacted by issues with that cloud, while a multi-zone setup spanning a region is generally "soft-guaranteed" to be resilient to normal operations / failures.