Another reason for learning it: you don't need to ask people to modify the cut&paste scripts you find on the web.
From a web dev point of view, if you are going back-end (i.e. PHP/mySQL/servers/etc.) then JScript is a nice little add-on just for reference.
I suppose it depends on your definition of learning it.
My approach is thus:
My approach is based on the fact that, at the moment I don't need to know it indepth. But knowing the basics comes in really handy.