This a very debatable topic. I think the main advantage of Struts2, compared to JSF, is its simplicity, it is much easier to grasp. Specially for beginners in web programming, or those familiar with "action based" web frameworks (Struts 1, Webworks, Spring MVC, Stripes).
But Lars Tackmann's points are valid (though Struts2 does not force you to use JSP; and indeed it uses FreeMarker internally). It's true that Struts2 has not attained a big user base, the community (both users and developers) does not seem to be very active, and that is a big minus. JSF also has its shortcomings, I've heard (I've not used it)... as every Java web framework, it seems.