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.
And yeah, Spolsky did talk about the perils of Java schools here: http://www.joelonsoftware.com/articles/ThePerilsofJavaSchool... Great article!
Having a single guy work in X can be bad indeed, but if you want to try something new its much safer than switching entire departments overnight.
I've seen companies of 300-500 employees struggle to keep even a single senior programmer over half a decade. They'd get far more talent by actually embracing an engineering culture. What I always see is that the juniors start getting experience, realize the place they work at is a huge pop culture and leave for an engineering one, thus lowering the talent pool of said company.
Basically they don't want smart engineers, they want replaceable code monkeys. They have yet to realize there's a few orders of magnitude worth of quality and productivity between the two.
I used to work in such an environment until I had enough of this cancer and decided to move to a small studio with less than 30 employees but with a very strong engineering culture.