Bananas are flagged as a risky item by a number of paleo-type diet authors, though not by 'The Paleo Diet'. They have a fairly high glycemic index. Not that it invalidates your point.

I lost 45lbs on the Paleo Diet and have kept most of it off 2 years later (I've crept up by about 8-10lbs and I'm trying to be a bit stricter to bring that back down). I didn't avoid bananas completely but I'd read enough to be wary of them. I'm sure it doesn't work for everybody but I find it persuasive and effective.

IF the theory that sugar and refined carbohydrates are the biggest risk factors for weight gain is true THEN learning enough to become convinced of just how bad they are, to the point that you develop a strong negative emotional response to foods containing them, is an effective technique of applying initially conscious rationality to create new habits. Of course it may not work for everybody. Variations in individual metabolism seem to be an understudied aspect of diet research.