DougRI think your remarks about the different status of Amazing Grace and Abide With Me in "folk music" are a very good illustration of the difference between "folk" and "folkies".
It's true that "Amazing Grace" has been widely recorded commercially in the last 20 years or so and that it seems to be sung quite frequently in pub sessions, including my own. However, before the recent commercial revival, it was nearly unknown to most people (including "folkies"). By contrast, almost everyone in my parents' generation in this part of the UK (around Cambridgeshire) appear to know all the 4 verses usually printed in the hymn books by heart and can/will always sing it when asked. It is sung at funerals, as an evening hymn and - in my family at least - as a "travelling" song (i.e. we used to sing it together when going somewhere - either walking or in the car).
Between the 1920s and the 1970s, "Abide With Me" was always sung in the community singing before the (UK) FA Cup Final at Wembley. Up to 100,000 people singing it was quite impressive.
By way of reference to the Alzheimers thread, I recall vividly the last time I sang "Abide With Me" with my dad. He had been diagnosed as having a terminal brain cancer which had reduced his speech to single words and strings of 2 or 3 now and again. We were watching "Songs of Praise" (Sunday evening hymn service on BBC). When "Abide With Me" came on he started singing it and, to his and our surprise, sang the whole thing perfectly. A great joy, but also the occasion of a few tears under the circumstances.
I wouldn't sing "Abide With Me" in a pub session as I think it wouldn't really be acceptable, though I do sing "What a Friend We Have in Jesus", which is tolerated.