True, JSP has some templating capabilities, but the biggest disadvantage of using JSP in JSF is that JSP writes to the response as soon as it encounters template text content, while JSF would like to do some pre/post processing with it. In JSF 1.0/1.1 the following JSF code
<h:outputText value="first"/> second <h:outputText value="third"/> fourth
would produce
second fourth first third
This was during the JSF 1.0/1.1 ages a headache. Developers would need to wrap template text like second and fourth in the above example in <f:verbatim> tags over all place. JSF 1.2 has solved it with an improved view handler which parses the JSP instead of executing it, but it was under the hoods still very clumsy as JSP syntax isn’t “well-formed” like XML. A XML based view technology was strongly desired, so that an efficient SAX based parser could be used. And Facelets was born (as well as Ken Paulsen’s “JSFTemplating” but ultimately Facelets was chosen as the new standard view technology for JSF 2.0).
Also, unified EL #{} couldn’t be used in JSP template text, resulting in ugly —and for starters unintuitive— mixing of ${} and #{}. Also, JSTL could in JSF 1.x on JSP not be used as view build time tags. Also, JSP syntax with <% %> things is old school and the possibility of embedding raw Java code in JSP is considered a very poor practice which breaks MVC ideology.
All with all, in JSF/MVC perspective, JSP is simply ugly and terrible and Facelets is simply clean and awesome.