We do not use 'popular melodies' for every improvisation, far from it! As I mentioned in the first post, our Director of Music frowns upon such things. But here in New Zealand, we do find it more difficult than other places perhaps to encourage more younger organists and by showing the organ in a slightly different light by incorporating well-known tunes into services on the odd occasion, it shows people the diversity of the instrument. We have had themes of God Save the Queen for services such as the Queen's Jubilee Celebration and these have all gone down very well!
To be able to improvise is a wonderful gift which not everybody can do and I feel that by being able to also use these popular tunes, we are drawing the organ closer to the congregation since they can directly relate to what is being played, instead of, perhaps, a Franck Chorale which, although absolutely stunning music which the average listener may indeed enjoy, is not something they can 'hum along to'! I do believe that this art does have a time and place where it is very appropriate to be played...!