This is a great thread and there are many valid points. I own (or owned) five fantastic cro-moly frames, IRO Mark V, IRO Group Buy, On One Inbred, Surly Karate Monkey, and the banger, a 94 full rigid Stumpjumper (still have it too). I have attempted to destroy each of these bikes and failed miserably. I'm pretty sure that any of these frames, assembled one tier up the cro-moly tube spectrum would be indistinguishable. No difference in quality of ride. Maybe a Snickers bar in weight difference. The Stumpy has supposedly fancy Tange Prestige tubing, but that is 1994 tubing! The technology to make great steel tubes for has existed for years, and yet people still go back to tried and true 4130 (Tange Prestige label says 4130). The Surly guys (and gals), Tony, Brant, and those jokers at Specialized know that building a good and durable frame comes from designing the angles, spec'ing appropriate diameters, wall thickness and butting profiles. Airplanes are made with this stuff, and who is a bigger weight weenie than an airplane manufacturer?
Think about the real world things, like ease of assembly (welding temps), dent resistance and availability of alloying materials and transport costs. It's too good not to use. Save the fancy tubesets for race day or full custom rigs. And if you think Surly, On-One or whoever is too expensive, buy one used, there are only about a billion of them everyday on craigslist and e-bay. On a daily driver you'll never know the difference!