By all means, MtheGm, but don't you also think that there's a further critical faculty involved, and that is that the 'mood' of the tune should fit the emotion of the song?
For example, I could just about imagine squeezing "Son David" into Jeannie Robertson's tune for "The Gypsy Laddie" - but even if I adapted it to a more solemn pace (which I might think appropriate), I suspect it would sound horribly wrong and far too major in mood/not modal enough.
(Please note, I haven't got to grips with the niceties of musical theory, but I think my ear has 'tuned in' over many years and my own musical preferences have clarified.)