I don’t think you can be dogmatic either way about this issue. Many roads lead to Rome. Even Charlie mentioned that he didn’t consider lunges, split squats or step-ups to be single leg lifts because they are supported on both legs. Personally I find that once you reach a certain level of maximal strength, you can afford to shift the emphasis to some other options like the step-up or split squat as long as you maintain your strength on the squat (perhaps as little as once every 10-14 days). We’ve seen Asafa do just fine with the exclusion of the squat and even Pfaff mentioned in his latest seminar that the squat was not the end-all-be-all that many make it out to be. He mentioned that a high box step up can actually be as effective and more challenging. I believe his leg series replaces the squat in his program if that is the decision made.
Speaking from personal experience I really do like squats, however I’m not really interested in going over a 5RM ever again as I’m not convinced that it’s necessary to continue to improve. Additionally I’ve found that high volumes of olympic lifts matched with a leg series of 2-3 exercises such as step-ups, lunges, weighted jumps, etc is just as effective, if not more so.
I’ll keep both in my program however as both options have worked for me in the past.