I can see what your getting at but I think this applies beyond programming languages. If you're interviewing someone who is into a niche programming language it can be a sign they are more interested in programming than say, someone who only knows Java etc. Obviously this won't always be the case and can lead to a weird sort of credentialing where people pretend to be into obscure languages.

It would be like if you were interviewing a chef who was saying "yeah yeah pan frying is fine but let me tell you about sous vide."

Then you go on to interview another guy who is into working the grill at chain restaurants.

Is the Sous Vide guy better? Certainly not always. But it can be an indicator this guy is a real nerd about cooking in a way that results in a better meal at the end.

Sure, it's definitely something worth taking into consideration, I just wouldn't value it too highly and in many cases I think there are much better indicators of ability.

>Learning languages is relatively simple and enjoyable, especially when the overwhelming majority of those people never end up working on difficult problems in said language.

Learning languages is not simple and enjoyable for everyone, especially not when they're languages with a radically different paradigm. I will definitely look twice at a JavaScript programmer who says they're also interested in ATS and Forth, for example. It should be fairly straightforward to suss out a rough level of understanding, and beyond whether or not learning languages is easy, it's a fairly strong signal of intrinsic interest, which a lot of employers want from their employees.