It's a sick, perverse, stupid practice if the frame is of decent quality. The bolt-on derailleur "claw"/hanger on my 1971 Nishiki always gave me an inferiority complex, since almost every other double-butted CrMo or MnMo frame had an integral hanger. Nishiki finally got it right a few years later.

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Disclosure: I hate single-speed. I have no interest in fixed gear. I really like derailleurs and consider what Frank Berto calls "gear phreaking" and gear changing itself to be a vital part of the cycling experience.