Having worked with both together for the past year I think combining shale and myfaces would be a good thing. I have alot invested in both, my team having just finished a large production system. I think clay is underrated and I would hate to see it dropped completely in favor of facelets. Clay's inheritance features have saved us alot of work, and I don't see facelets supporting this feature. Also, my team has been able to encapsulate many of our common application fragments using clay (1). Not sure if this could have been accomplished as easily using facelets.
(1) http://wiki.apache.org/shale/ReusableClayJars
