Loie and I have seen several examples of churches incorporating older materials or sites. In Brittany: a church built on a hilltop, with a small megalith a few yards outside the front, in line with the path to a stone enclosed spring; a church with a small crypt that was originally a neolithic dolmen. In Paris: a church with two excavated Roman Era pillars inside its front wall. In Italy, near Rome: a church built on the foundation of a Roman era temple of Diana.

Some of these few random examples could be chalked up to practicality, others don't seem to be so amenable to such an explanation. Hmmm.