You're selecting for language dilettantes, then, I'm afraid. Especially for languages like C++ or JS, that contains worlds of different approaches within their umbrellas, I don't necessarily see not dabbling with something that fills the same niche as a deal-breaker, or indeed all that valuable, really.

I would consider a more useful filter looking at people that have experience in multiple levels of the stack, whatever your particular niche happens to be. For web dev, having a good handle on two or more of: SQL, a server-side language, and JS/CSS/HTML. Substitute whatever blocks fit in your particular equation.

We're talking about two different types of programmer I think. You're talking about a maintenance programmer and I'm talking about someone who might be able to move the technology stack forward and possibly save some money in the process by finding a new and more efficient way of programming. I wouldn't expect someone who is happy doing the same thing over and over again in maintenance mode to be that person, although this type of programmer is a very necessary part of the team.

It might be a function of team size. I've spent most of my career in small teams, and so the flexibility to wear a couple hats at a time is relatively more important, and helps reduce the bus factor.

Would you give a pass for Purescript or Elm?

Those are great too. Something that shows a passion for learning. How could a passionate JavaScript programmer not learn Dart, TypeScript, or Elm? Even if to say why they're no good.