There's an exception to that: most European microstates adopted the Euro but aren't part of the Eurozone (nor the EU) and have no power in decisions made about it. Of course I concede that microstates have unique properties that break away from most considerations that are applicable to "typical" sovereign nations but it's not as clear-cut as you describe.

I'd argue this isn't inconsistent; these states are so small they effectively have ceded control of their economy, as well as their national defense, to the larger European community. Kosovo and Montenegro, for example, previously used the German Mark. This seems pretty close to an admission that for whatever reason (scale, stability) they're not able to issue their own currency in a credible manner.