The idea that everything good belongs inside the Christian way of life is a very important strand in Christianity over the ages. Not always the dominant strand, and that's the pity of it, but the places and times where it has are the places and times which are closest to being Christian, or somit seems to me.

"The song is only nominally a hymn to Mary, Mother of God" - of course it is precisely that. Composed with that in mind, sung with that in mind, in celebrations centring on that. And responding to the same human instinct as some pre-Christian traditions responded to.

"...it would be nice if one belief system didn't require the absolute abandonment of the another." Well, it is nice, now and then. More often than you'd think, in fact. We know the world is round, but we still use draw maps drawn on flat bits of paper to find our way about the place.