On the other hand, if you are only half-a-rationalist, you can easily do worse with more knowledge. I recall a lovely experiment which showed that politically opinionated students with more knowledge of the issues reacted less to incongruent evidence, because they had more ammunition with which to counter-argue only incongruent evidence.

What exactly is the problem with this? The more knowledge I have, the smaller a weighting I place on any new piece of data.