To me she's a great and unique singer. One comment I would add, is that "folk" can approached from many angles, and that classical or mainstream pop vocal styles are not the only other alternatives.

In my case before discovering English folk I was a fan of mainly indie/alternative/rock music and have always appreciated an expressive and interesting singer over one with a bland but technically perfect voice. To me Shirley Collins singing is beautiful and sums up what I love most about the music.

I know taste is a personal thing, but criticism of her as someone who "can't sing" simply because she doesn't meet certain peoples preconceptions of what a "good" singer should sound like feels like the type competitive judgemental "muso" wankery that seems to exist across many genres and has always seemed to me like the wrong approach to music.