I think what we see here with certain people's adherence to the MBTI types is very similar to what happens with horoscopes: "I like what I read and it is vague and general enough that it absolutely can apply to me, so by god, it's right!" I mean, my MBTI type is "mastermind." It's very flattering to think I"m a rare test result and come on, a name like mastermind? I could go on and on bragging about my type if I wanted to. But it's meaningless. I don't. I agree with all statements about confirmation bias and wanting to see in yourself what you read in your result. It's flattery, plain and simple.
I agree, when people take these tests they expect their answers to come out the way they want them to. Although I don't think that this necessarily disproves the validity of the test, because it still makes you look for what you believe to be true about yourself. I know myself to be an introvert, so when my results came out as INTP/INFP, I found it interesting to see things that seemed to fit my personality type. Flattery-maybe. Untrue-not entirely.
Cognitive dissonance as well. It's a line of thinking that fits with people's current perspective and thus very easy to accept. If the test would say stuff that wasn't true, it would be far easier to dismiss. But the test has a good category for everyone, there's no 'bad' category.I agree with all statements about confirmation bias and wanting to see in yourself what you read in your result. It's flattery, plain and simple.