It's important to note, because it's often missed, that etcd can operate either as a participant in the high reliability / consensus portion of the cluster, but also as a proxy that does not participate. So you don't have to think in terms of 50%+1 of an entire cluster needing to stay up for consistency, only 50%+1 of the etcd cluster. This is nicely illustrated in the the cluster architecture docs at CoreOS.com.

https://coreos.com/os/docs/latest/cluster-architectures.html