In the 1970s, I very much enjoyed working through Jean Sammet's classic book "PROGRAMMING LANGUAGES: History and Fundamentals". For one thing, it was my first real exposure to Lisp, and it was fun learning many new languages at once. At one point I had over a dozen programming languages on my resume (due to needing to use some one-off languages for military hardware, and several different assembler languages).
Generalization is good because the more experience with different languages, libraries, frameworks, and development styles that you have, then the easier it is to choose a good technology to solve new problems. I would argue that broad experience is at least a little better than narrow but deep experience.
Unfortunately there is another side to this issue: whenever I see really great design and code, it almost always seems to be written by someone who deeply specializes in one, or perhaps two, programming languages.
In the last few years, due to customer requirements, I have had to work in Java, Common Lisp, Ruby, and Python (and a little work with Prolog, and even less fortunately in C++). I am now in my mid-50s, and semi-retired (I try to limit myself to working no more than 25 hours per week). I would like to specialize in just one or two languages and not have the overhead of staying current with a half dozen programming languages.
The language that I enjoy the most is Ruby, and with the maturation of JRuby, it is great to be able to use the same language for scripting and general software development (using Matz's C based Ruby) and when I need to use existing Java libraries and frameworks (JRuby). While I really enjoy Lisp and Smalltalk (never professionally), my hope is that as JRuby continues to get faster and integration with the Java platform improves that the combination of (C or J)Ruby and Java will cover everything that I need for my work. BTW, the comment about future work on a "compiler that allows compiling a given Ruby class 1:1 to a Java class, producing a class you can construct, with specific signatures you can call from Java code" posted by Charles Nutter a few days ago to my JRuby blog article was very good news indeed.