[marssaxman]

It is not obvious to me; in fact I would argue the opposite. Each new language generally has an interesting new idea or two, and learning a new language can expand your conceptual toolbox even if you never write enough code in it to consider yourself a master.

I don't know how I'd count the languages I've used so far, but it is certainly more than ten. I'm not fluent in all of them, but almost every one has taught me something. For the most extreme example, I still can't even read Haskell code, and I've never written a lick of it, but in reading about Haskell I picked up some ideas which have had a profound influence on the way I write code in other languages.

One of the reasons I am not a fan of Javascript is that I didn't learn anything from it. All of its bits were obviously cribbed together in haste from other languages. This is no slight on Brendan Eich, who was given something like two weeks to design it, but it's a shame that the most widely used programming language of all time has to be such a mess.