The big thing here is cost.
> If you put the above rules together, you can see that the minimum MySQL deployment is four servers: two in each of two colos...
The ideal scenario is to have 4 "fully equipped" nodes, 2 in each data center. That means having 3 pieces of expensive "by the hour" hardware sitting around doing basically nothing. (and paying 4-5k / computer for MongoDB licenses)
In that scenario you can have everything on instance store and live with 4 copies on volatile storage.
Of course, no start-up wants to commit that many resources to a project. It's far cheaper just to use EBS and assume that the data there is "safe". Is it bad practice, would I avoid EBS like the plague? You bet!
But it's definitely cheaper and that's hard to beat.
Chris Westin, of 10gen, blogged about this a while ago: https://www.bookofbrilliantthings.com/blog/what-is-amazon-eb...
In fairness though, 10gen's official stance is to use EBS. I think that's a mistake, and I think maybe they do it for extra safety.