I don't know anything about LD, but a steel 72 does not hold much less air than an Al 80 (which is actually holding around 77-78 cu. ft). So your remark about this being a way to give shorter dives does not really stand up.
In practice, that extra few cubic feet is going to be trumped by many other factors of a dive operator.
I will lightly differ with you on your opinion that a 72 cubic foot cylinder is no different than an 80 cubic foot cylinder. You are entitled to your opinion so I am not going to go into that argument that eventually go to Rock Bottom Gas Management and agency "A" standards bashing.....
I would however propose that using the small volume cylinder might provide some convienence by lightening the surface load on the divers, using less storage space and making it eaisier to move around the boat with your gear on. These reasons I do like.