I think that by leaving the stones visible from the outside it shows some respect to the heathens/pagans.
They can touch the stones before coming into the church and being indoctrinated.
When I was young I was told that churchs with equal height spires or a single spire were for Xtians only, those with one spire smaller were tolerant of heathens at certain times of the year and those with one spire higher were heathen friendly at all times.
I have a lifelong passion for gargoyles and hunkypunks and pews end which often have the most amazing carvings.
PeteG