For me, I really don't regret learning any of them, and they all had something to teach me that I've carried forward.
For example, learning Common Lisp really let me internalize the concept of programming a meta-language to make my actual job easier (I later went on at that same job to write a very large, sophisticated code generator for a lot of humdrum deserializers). Learning Erlang helped me realize what a distributed system really is. Learning Java helped me understand what good concurrent threaded programs based around semaphores and queues could really god. Learning C# helped me realize there was a whole another level than that. Learning Clojure helped me realize we could use STM in the real world.
I could go on. But all of these things are things I carry with me to this day. I don't do it for "nerd cred." I do it to grow as a software engineer.