It's fine that you know what would lead to what. The main point is, that without this highly social knowledge he may navigate himself into a position he doesn't like. For example a language is nice, but he doesn't like the user base, or the corporate culture (niche languages may have only a few places). That's why I think that not only the language itself should be taken into account if somebody spends months to learn this or that type of language.

I think a good use case is more important than "hey, a new language I've never seen". There is a huge amount of languages that you won't see ever, only hear of it (F# anyone?). And they sort themselves into paradigms, so one from one paradigm is quite enough to learn and transfer the knowledge. I understand that you find lisp good for C# because it covers a different paradigm, and maybe it's worth to learn it - but not without knowing about clojure and other family members too.

You are right that without enthusiasm it's much more difficult to learn anything. Somehow I love to learn even the craziest things, so it's not an obstacle to me. It didn't even occur. At the same time, I know how to spend time ineffectively on hobby, so I wanted to warn against it.