Syag, why does this information about Carebach disappoint you more than knowing MBD used non-Jewish tunes?
I see two differences. One, I don’t think MBD ever claimed that he never “borrowed” secular tunes, but according to oshriv, Carlebach did make such a claim – in fact he claimed he never listened to it. Two, the big Carlebach people like to think that his music is 100% authentic Jewish music, not influenced by anything else. It’s simply not true; he certainly borrowed elements of his style from folk music and other styles.
(answering for myself)