Yes, that, and the quality of applicants tends to be higher for some of the less popular but more interesting languages. I worked at a company that had a large java codebase and was reluctant to try anything different for fear of not being able to find developers. But then we'd get these applicants with dozens XML/JAX-WS related keywords on their resumes but who couldn't write a simple for loop or understand what a database transaction meant.

I recall reading about someone (was it Spolsky? IDR) who looked for candidates with certain niche languages on their resumes, because it served as a quality signal. If they're interested in learning Clojure, for example, they may just be more generally curious and/or have a desire and interest in learning about different paradigms.

That said, at a practical level one has to balance competing demands. If you have someone on your team working on a project in X and they're the only X developer you have, you're at risk because if they leave, you may be screwed. Plus, they won't be getting quality feedback on their work because no one else can understand X.