is taking (for example) the MBTI and accepting/rejecting bits of the result more valuable than, say, starting with the hypothesis that you are just like some fictional character and accepting/rejecting bits of that description?

Hmmm... I haven't tried the "fictional character" one, so "experience" isn't what I'm applying here. It sounds vaguely promising, except that fictional characters are not optimized in their traits or presentations to be informative in this way (being instead devised for story delivery, possibly unreliably narrated, presented in only specific situations and not at randomly selected moments, etc.), and the MBTI results at least make an attempt at being useful for this sort of exercise.