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You get other advantages for doing three/five node replication--online updates are free if you require (n-1) compatibility.

Reality is that if you need to hit 5+ nines and require strong consistency guarantees (like "we never lose our customer's data") at data center scale, you'll probably need something similar. Most people probably don't need those guarantees, however.

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I should have clarified -- I meant mostly for things like OLTP databases.

I see why Google built Megastore, Spanner, etc. And sure, ZooKeeper or etcd makes sense for putting small amounts of configuration data.

But most of us aren't Google, and synchronously replicating the entire OLTP database, 3 or 5 nodes times the number of shards, seems kind of absurdly expensive for most people.

A node can handle many more than one shard. So you can basically have 10 nodes, 8 shards with 5 replicas each. (Just an example)

Oops, you are correct -- I somehow forgot that shards are logical, not physical.

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