"You have brain damage" is also a theory with perfect explanatory adequacy. If one were to explain the Capgras delusion to Capgras patients, it would provide just as good an explanation for their odd reactions as the imposter hypothesis. Although the patient might not be able to appreciate its decreased complexity, they should at least remain indifferent between the two hypotheses. I've never read of any formal study of this, but given that someone must have tried explaining the Capgras delusion to Capgras patients I'm going to assume it doesn't work. Why not?

Maybe it's really hard to really get that you are a brain on an intuitive level. Human intuitions seem to be pretty dualistic (well, at least mine do). So 'you have brain damage' doesn't sound very explanatory unless you've spent lot of time convincing yourself that it should.

By the way, the last link is broken.