I've always thought satiety was the main reason for increased protein on a low carb diet (though the New Atkins authors refer to adequate protein rather than increased), rather than preservation of lean mass (I see that as more of a by-product). Of course it all comes down to the question: How much protein do we really need? And are we looking for minimal performance or optimal? And what variables can change the amount we need? I see a lot of speculation around these questions and not a lot of real research.

I do recall a study from a couple of years ago by George Bray, I believe, where he found that dieters that ate more protein than the control group had better compliance and better results. I think he proposed to be comparing low carb vs low fat, but the low carb arm was not particularly low carb, and the low fat arm was not particularly low fat, but the increased protein group did well.