The belief of these people is usually that fat+carbs+protein produces more postprandial insulin than eating carbs alone or eating protein and fat together without carbs, and then the insulin stays high and does damage/makes people fat. But eating like this is only a bad thing for the metabolically deranged (high blood glucose and insulin, poor insulin sensitivity). Those with good insulin sensitivity don't produce all that much insulin and what they do produce goes down expediently.
As for correcting metabolic derangement, has anyone truly corrected it with a vegetarian diet or do they all complain that the second they try to eat a mixed meal with meat they have convulsions? What about Atkins-Taubes style low carb? Same deal. What's the solution? Fix the pathology. I would say that it is easier to do with a low carb diet than one with more carbs, and if done right then they can come off of the extreme carb restriction if they want to. Many choose not to though, which is fine so long as they are truly healthy and not just using macronutrient restriction as a band-aide.
Stabbing conventional wisdom in its face.
Anyone who wants to talk nutrition should PM me!