IMHO learning about a new language (without becoming proficient with it) is always good as it makes you think about the one(s) you're proficient with.
Mostly true, but there are languages which are completely useless without becoming proficient with them. In my experience Forth and J are such languages: you can "learn about" them, but you won't really understand them without using them for some time. So I suspect APL, K, Q and Eli here are similar; the question is what will I gain from becoming proficient with one of them or alternatively is it worth becoming proficient in one of them if I'm already proficient with another.