I don’t wish to add (much) to the good responses that have appeared already (see Andrew Anthony, Norm Geras, David Thompson and Oliver Kamm); I’ll just note one thing.
Edgar says that he’s “interested in the politics of defection”, although he seems to be more fascinated by the psychology of defection (or rather, the psychology of changing your mind when political cliques of some sort are involved). Key to his view is this:
Inevitably, however complete the conversion, what defectors think and do now is coloured by what they thought and did before.
Well, yes, that’s probably true. But you can also make a perfectly good psychological case with which to loftily sneer at consistency of view.
If my now thinking ‘X is bad’ is inevitably coloured by my having once thought ‘X is good’, then of course your now thinking ‘X is good’ is even more so coloured by your record of having thought ‘X is good’ all along. Dare you not be self-critical? Do you magnify, deny or simplify new developments so as to justify keeping your old position? Do you refuse to see things in new ways? Have you invested so much of your life in slogans and tracts and comradeships that to break with that would be more harmful to you than a mere admission of error?
We all have pasts; we can never escape them; our futures will always be coloured by them. Welcome to the human race.