Hello fellow JDeveloper developer!
I have been working with JSPX pages for over two years and I never had any problems with them being JSPX opposed to JSP. The choice for me to go with JSPX was kinda forced since I use JHeadstart to automatically generate ADF Faces pages and by default, JHeadstart generates everything in JSPX.
JSPX specifies that the document has to be a well-formed XML document. This allows stuff to properly and efficiently parse it. I have heard developers say that this helps your pages be more 'future proof' opposed to JSP.