When I was a boy scout - rather a long time ago - and learnt basic map
reading - we were told that churches were on the map (the good old one inch
map) because they were landmarks that could be seen from a long distance -
hence the need to distinguish those with spires from those with towers. I
was never sure how helpful this was in a town - though I suppose prominent
churches are still useful landmarks even in a built-up area.
One can see that an argument could be made on this 'landmark' basis (like
electricity pylons) whether or not the 'place of worship' now functions as
one or not. I'm not sure you can argue they should be included for
historical reasons - unless it's as tourist attractions. Can you argue for
inclusion alongside telephone boxes and POs and car-parks as useful public
facilities? - would you be trying to find the nearest functioning 'place of
worship' in an emergency? If so we would probably need different symbols to
distinguish e.g. synagogues from Quaker meeting houses.