I think that, very basically, when it comes to ideas rationalists explicitly don't have anything to protect. Ideas are to be judged by their merits without interference. This has to include the Something to Protect that brought about rationality in the first place, because to the degree that thing isn't rational there is a contradiction in using rationality to protect irrationality, the defensive attitude and bias you mentioned.

Can "definite major purpose" avoid that problem (beyond sounding unlike what is meant)? I'd shorten it to "major purpose" or make it "prime directive" or "main quest" just to avoid anything definite. It should be subject to change with new information or better thinking while the rational methods used to achieve it stay the same.