Whoa up here Kemosabe........You can run an entire thread on any number of singers such as Dylan and Dave Van Ronk and quite a few others who are damn near non-understandable at times but have a magical ability to deliver a song.
Or there's Option #2........What about the singers who sing in an a dialect of English or "American" where marginal diction is actually a part of the dialect? As an example, let's take Frank Proffitt. If you "clean up" some of his words, it ain't Frank Proffitt! On the other hand, if you try to imitate him you lose on that count too. You can only do the song justice though by letting your own voice carry the words. Somehow though, I have a real hard time with Placido Domingo singing "Blackberry Wine" accompanied by the Cleveland Symphony.
My point is that diction may be important, but it doesn't always make the singer good or help in delivering the song.
Spaw