It should be qualified that you should learn languages that are actually conceptually different. You could easily learn 20 languages that are practically identical as far as the actual concepts are concerned. The difference, to you as a programmer, between C and Python, is much smaller than between Haskell and either of those, or between Haskell and Idris.
Covering a larger volume of concepts is what should be the emphasis. That is definitely useful, maybe even more useful than just memorizing stock algorithms and data structures, especially if you engage with the mathematics behind the languages.
Agreed. For example, I spent a couple years using JavaScript as my main language at work then changed jobs and learned OCaml. If I need to use JavaScript again, I'll do it in a much different (IMO better) way than before I understood functional programming.
Covering a larger volume of concepts is what should be the emphasis. That is definitely useful, maybe even more useful than just memorizing stock algorithms and data structures, especially if you engage with the mathematics behind the languages.