On the other hand, it's quite a reasonable assumption that getting a good understanding of JavaScript gives a broader context of what is going on that is necessary to properly use jQuery.
Second, the argument that language X runs language Y under the hood so you should learn Y in order to get a good understanding of X doesn't hold. JavaScript is written in C++ in most cases, which compiles into assembly. So why not learn assembly to get a better understanding of JavaScript?
That's silly.
With all that said, I also lean towards "just learn what you want or need to know" train of thought. I think those of us who enjoy learning what's under the hood overestimate it's necessity for "getting things done". I don't think there's anything wrong with learning and using jQuery without diving into the messier details of vanilla Javascript unless necessary. Also, for many of us, those ugly situations when we are forced to figure out what's going on behind the scenes is when we actually understand it anyway.