My brother is a very good ColdFusion programmer. He got his start in the language because it was what I was programming with at the time. (Don't forget, around '99 when I got my start, there was Perl, ASP and Coldfusion as serious contenders.)
I ask him, "Why don't you give PHP or Rails a whirl?" "No interest." And so he focuses his effort in refining his ColdFusion skillset. He does Ajax and does CSS design with the best of them.
I don't know any hacker who doesn't have an intense, irrepressible interest to learn new stuff - languages, frameworks, whatever. I also don't know a single hacker who hasn't learned well over half a dozen languages to a proficient level. And no, CSS and AJAX don't count (unless the Javascript is taken to a scary level). Sounds like your brother is a designer with some coding skills, not technical co-founder material.
Any programmer worth his/her salt should be always learning newer technologies that might be useful to him/her. Having a favorite is different than not wanting to learn anything else.