The JSTL and EL go hand-in-hand to eliminate the need for messy and old-fashioned scriptlets in JSP pages. In fact, the JSTL without the EL would be pretty useless.
Also, as Bear mentioned, EL is been used in all taglibs I know about (JSTL, JSF, Struts, etcetera). So I can't imagine how I would use taglibs without EL.