I was thinking only the other day about critics vs. musicians - has there been many/any good critic-practitioners in music?
loads.
many classical and 20th Century avant-garde composers were critics and essayists and theorists - Wagner, Schoenberg, John Cage, Michel Chion, Pierre Schaeffer, Pauline Oliveros, Michael Nyman, lots more...
and often they were in fruitful dialogue with critics - Wagner and Nietzche (who also did a bit of composing himself) had an intense relationship, extremely mutually warm at first(Nietzche moved in with RW and his bird Cosima) but ultimately leading to a parting of the ways (whereupon Nietszche wrote no less than three anti-Wagner books!)
Eno is a paradigm example of someone whose thoughts on music (and culture generally) ought to be compiled into a book (his Diary With Swollen Appendices is great fun but isn't quite that)
Green Gartside, Momus, David Byrne, Drew from Matmos.... all could have / should have /actually have written books
But do you mean critics who are primarily or initially only critics, and then move into music?
there's quite a few examples of that syndrome - Greg Tate with his black rock group Burnt Sugar, Paul Morley with Art of Noise (in a sense) and Infantjoy, Patti Smith and Lenny Kaye, more that i'm blanking on
at one point i was going to write an article on the phenomenon