Thats a good point Mr W.
Carved stones have been recycled since...well I guess, since people have carved stones. Famous example include Knowth, Newgrange, Camster and a good number of Breton monuments. The Knowth carved stones were not only recycled into the later tomb but also incorporated into a 9-10th century souterrain. We also see many examples of carved stones incorporated into the kerbs and mounds of BA monuments
http://www.themodernantiquarian.com/post/34417.
I guess, for whatever reason, we have always loved these carvings.
I particularly like this recycled stone (-:
http://www.themodernantiquarian.com/post/48351