I completely agree. This is a blog post I wrote on the subject: http://robotr.tumblr.com/post/189911981/following-trends-wha... (but you do a great work telling this briefly)
Lisp lets you to see your source in different light. Haskell makes you rethink expressions. C lets you know how it all works under the hood. Factor makes you think both directions.
There are also languages that set the bar high for you. Ruby for elegance. Prototype or jquery for making complex ugly thing a paradise. Even Perl's CPAN shows you the value of cooperation and modularization.
Even bad languages teach you a lot about ugliness, bad decisions and denial.
The same can be said about quite a few frameworks or even tools. strace makes programs transparent, for example, while tcpdump makes you see, you realize you were blind.
As for math and algorithms, you suddently know enough of them, they're pretty finite.
Imagine two paths for a developer:
- learn a lot of languages, like everyone else does... a lot of this learning will be useless with time
- learn, for example, a lot about computer vision... the road for this is not "pretty finite" and as you learn more about this, you can work on more and more impressive projects every time (making your value bigger)
This is how I see it, and I don't think the first path is worthwhile. (Yes, I learned a lot of languages and frameworks too... but at a point I started asking to myself: why care about most of it? the real stuff is not this)
I think the Linus Torvalds example was a good one. Or John Carmack. Do you see them talking about a lot of languages and frameworks or do you see them learning and doing new stuff?