Should probably clarify that it's only important if you're looking for Web jobs :-) Sometimes people here forget there's more to software than the web.
I'm very much more on the data analysis side of things and decidedly not a web developer, but I still regularly get told "great work, can you display the data on a cool website?" In fact if I was hiring a new data analysis colleague I would very much prefer one who knew javascript for this very reason.
Until a week ago, the only reason I needed to know Javascript was for my own side projects. I have never done anything requiring the web professionally until about 4 days ago.