One concern can be that talking in theory prevents the client from talking about their real life and struggles, in real terms that are meaningful to them.So discussing theory can be a way of avoiding painful topics that are much closer to home than readings, science, etc. That is the explanation a professor I had in grad school gave. Personally, though, I believe that a good therapist knows how to tactfully help you back to what's relevant for you at a pace that is adequate. I also think this explanation may be prejudicial to some clients. I know people whom books have helped a lot in their self-discovery and growth and I think it would be counter-productive not to talk about those books if it's helpful.