I've noticed that people who are known as great guitar players, but also do lead vocals, are criticized as, "he should just stick to guitar playing", etc., And in most cases, their singing sounds great. I've heard these criticisms of Joe Bonamassa, Joe Satriani, Yngwie Malmsteen, Paul Gilbert, and a few others. But upon listening to them, they don't sound any worse than any other famous singers. I remember a goof made by a radio station a long time ago, where a DJ was criticizing David Gilmour's vocals, saying, "Roger Waters was a better singer. Gilmour should stick to guitar playing because this new Pink Floyd stuff doesn't sound very good", not realizing that David Gilmour sang most of the early Pink Floyd stuff. I've noticed a similar phenomenon with people criticizing directors who perform cameos in their own movies. Quentin Tarantino's cameo in Pulp Fiction was pretty awesome and funny, but the comments are the along the lines of, "He should stick to directing, he can't act.".