While I'm not familiar with Spanner's inner workings, I would guess that they recommend 3 instances for quorum establishment in case a region becomes unreachable. If that's the case, using fewer than 3 instances could cause major problems.
Since Spanner currently only supports single-region deployments, it clearly isn't recommended as protection against a region becoming unavailable.
It may be recommended as protection against an availability issue on an instance, though, which is, after all, a big reason why you'd want a distributed DB in production.
I suppose the loss of a region doesn't apply (yet), but yes, the quorum requirement would still apply even if you only had instances in a single region.
