Alistair Cockburn wrote

Agile development calls for a certain amount of ambiguity and flux in the project. Not everyone enjoys ambiguity and flux. I would suggest that most people don’t.

A very good point.  I think this affects some agile implementations – causing them to back away from being really agile, into a no-man’s-land between agile and waterfall. 

Personally, I prefer the honest ambiguity of agile to the Clayton’s certainty of waterfall.  Software development is risky.  Customers and suppliers do learn during the project.  Users do give better feedback from trying software than reading documents. To pretend otherwise may feel safer in the short term, but it disappoints in the long run.