I would have expected an example if you could prove your point.
Anyways, seems you got stuck on my example of user story which I think I didnt put in right way before you.
I was trying to highlight the interdependencies between various stories & tasks. It would be naive to imagine stories having no linkage or dependancy on previous stories, in which case it would be mistake to not come with a good estimate.CPM helps come with the most realisitic schedules in such situations. I agree, it doesnt happen much often that one story has dependancy on 3-4 preceding stories but ofcourse, I am not talking about school projects but complex applications developed in fast-pace ,big-money , high-risk environments.