A QT is as good (or as bad) as the hobbyist made it. If the QT has good environment, well maintained, quality water levels, then thank the hobbyist. If, on the other hand, the QT has poor water quality, is too small, is poorly maintained, then blame the hobbyist, not the practice of quarantining (which is a totally sound practice). Likely, the individual who has a bad QT, also has a bad fish tank.