Struts ServiceManager and Struts PlugIn

[dbgroups]

hello:

Would someone enlighten me on proper role of:
    1) Struts PlugIn: add features without the requirement of extending ActionServlet.
    2) Struts ServiceManager: extend Struts framework without the need to extend 
ActionServlet.

What is the thinking of other add-ons such as Tiles?

Would ServiceManager be ready soon and/or for Struts 1.1?

Thanks.