They are generally tough, and can be made into a frame of reasonable, but not super light weight. They are a good choice for touring bikes, and economical track bikes, where stiffness is important and toughness is a plus. I like to build touring bikes stout, using tubing with larger diameters and thicker walls to avoid unwanted flex that loaded packs can cause. The extra mass that is added to the frame beefs up the strength and makes for a super stable bike. Extra wall thickness resists dings and affords more strength of structure.
Extra diameter increases stiffness.
The greater strength is achieved either by heat-treatment, or by varying te elements of composition, or both. Because of the higher strength of these tubes, for any given rider we can build a lighter bike that is as strong as a standard cromoly bike by using thinner-walled tubes. The desired stiffness is maintained by using the appropriate diameters. After 753 came the beautiful Tange Prestige to the high strength steel ranks. The superlight 0.6mm thick chainstays will be particularly missed.