I think John E explained it back on 08-23-03. There still seems to be confusion between rims intended for tubular tires, and the heavy duty double walled steel rims also called "tubular" for their construction, but were intended for clincher tires. The latter were common on lower end (think Schwinn Varsity, Raleigh three speed, etc) but decent quality bikes, too low end that weight was not a great consideration, and tended to be quite durable.