I work on Guile because I enjoy messing with compilers. I absolutely agree that very few projects use it (exceptions include Lilypond and GNUCash). However, that might change - Guile 2.0 switched from a simple interpreter to a virtual machine and a compiler to that VM. That means that Guile is competitive in speed with other scripting languages (and actually faster than some of them, I believe). It also means that it now supports multiple high-level languages. There is currently a good Emacs Lisp implementation and about half of an ECMAScript implementation.

I don't know if it'll become more widely used, but on the mailing lists you certainly see people writing libraries and contributing code, so I think there is a real chance of it. My sense is that Guile is now coming out of a period of stagnation. I don't know where it's going.