I strongly suspect that if I held a gun to Mr. Singer’s head and was about to splatter his gray matter against the nearest wall because his views made ME and most other reasonable persons UNhappy, that he would be whistling a different tune about his utilitarian “ethics”.
I think about that line of argument too when I hear certain smug academics and "intellectuals" assert that "usefulness" is the only criteria for being allowed to live. Well then, if I put a gun to the academic's noggin and threatened to end his life because I found a group of people who agreed he was worthless, do you think the academic might rethink his position? Based on usefulness, liberal arts academics are about the most useless people around.