Firstly, I disagree with your premise. I think JSTL is incredibly simple to learn as opposed to a rather large (and obtuse, in my opinion) framework like Struts.
The biggest hump in using JSTL effectively is wrapping your mind around the expression language -- which is something you'll need to do anyways since
JSP 2.0 is coming and the expression language will feature prominently.
Besides, Struts is a complete web application framework. Why use it if all you need are JSP tags?
bear