I agree that if it was possible to clamp immediately after tensioning, there would be no difference between a lockout and a constant pull. But I feel there is a significant difference, good or bad. the USRSA recommends an adjustment of almost 10% pulling tension to get the same approximate results between the two machines. And there is a difference in final tension and string diameter on a constant pull clamping at 2 seconds vs 10 seconds.
I like Max Ply's notion of keeping the distance between the frame and tension head consistent, but I think the lockout machine mitigates most of the "time to clamp" issue, which can make it very consistent. A good analogy might be an automatic transmission vs a manual.
I like having the control the constant pull gives me, although the potential for inconsistency seems higher. Natural gut and other strings that tend to display high initial elongation I leave tensioning longer before I clamp; poly I try to clamp as quickly as I think I can do consistently.