AFAIK, Prolog is even heavily in use in some large-scale custom software projects. It definitely has a niche that is quite hard to fill by languages like C++.

I had to learn some Prolog, Modula/2, ADA, ML, Scheme and Lisp in the first two years of my CS studies. In hindsight, I really appreciate the exposure; it has given me a good foundation to learn new languages and it has teached me many concepts that I wouldn't have learned that well if I had only learned Java or C++, for example.

Of course, these experiences also have led me to solidly dislike C/C++ *sigh*. I tend to think that too many apps are written in C++... makes me look forward to Parrot and Perl 6 even more!