The reason this rule exists is mostly because using "I" introduces the possibility of subjectivity, and opens up the argumentum ad hominem, both of can sometime

The reason this rule exists is mostly because using "I" introduces the possibility of subjectivity, and opens up the argumentum ad hominem, both of can sometimes make a sentence weaker. "You're 17 years old; what could you possibly know about the economic state of affairs in the Andalucia region of Spain? I'm going to go find a real expert, like the ones you've cited!" Along similar lines is the moratorium on rhetorical questions, which open up the possibility that the reader's answer will be different from the one you want them to have, at which point you've let the reader make up his/her mind before you've had a chance to change it. I think both bits of advice are useful sometimes (I particularly dislike rhetorical questions, as they usually feel like beating around the bush), or even most of the time, but I teach my students to avoid both (along with using "you" in papers) unless they can make a stronger argument with them (ethos is everything), in which case they should err on the side of making a stronger argument.