The purpose of the Persona, I believe, is to add empathetic focus to the design. Empathetic focus. By focus I mean that the design must be clean and coherent. It is not a collection of features added willy-nilly through the life-span of the product, even if each feature by itself makes sense. Rather it is having a clear image of what the product is meant to be -- and what it is not meant to be -- and rejecting features that do not fit, only accepting ones that do. By empathy, I mean an understanding of and identification with the user population, the better to ensure that they will be able to take advantage of the product, to use it readily and easily -- not with frustration but with pleasure.
-- Ad-Hoc Personas & Empathetic Focus
Jeffrey Veen terms those personas unbacked by research "The designer's imaginary friends." The article Human-Centered Design Considered Harmful, also by Norman, along with the inevitable HCD harmful? A Clarification.
One insight from my own observation was a fairly complicated CMS package users weren't posting with. My conclusion, based on observation, was the users were afraid of making mistakes which would reflect poorly on themselves. Self-image. Since redesigning the package wasn't an option, I created a tutorial and practice instance of the CMS users could use specifically to make mistakes anonymously.
This insight would not have been developed out of thin air. Rather user research must inform persona development. Yet many companies prefer to skip the research part.