We know from Fredo's "why didn't we ever spend time like this before" comment that Michael spent precious little time with him. This remark, in my mind, was Fredo's way of saying "this is all I ever needed from you", and "I would have looked at things differently had we done this before". Dreaming up specific tasks,responsibilities or reasons to engage Fredo was Michael's job, not mine. 20/20 Hindsight is not how the Don would have looked at the aftermath of Michael's carelessness. Wasn't there a lesson to be learned from Tessio's betrayal and the careful methodology that Vito walked Michael through to help him anticipate the weakness from within? Granted, it was done for different reasons, at different moments in time, but the underlying lesson was to anticpate and plan for the most or even least likely betrayal and attain 20/20 foresight.