I hadn't thought about it before until I saw this post by Kate Woodbury, but it's because blog posts contain a lot of the words "I" and "me."
Since "no first-person" inevitably results in bad writing (an overabundance of passive voice; the use of "one" or "student" instead of "I"), I always tell my students, "You may use first-person in my class. In other classes, check with the instructor."
I never thought much about WHY teachers were telling students this. I vaguely remember someone telling me not to use first-person, and I vaguely remember ignoring that someone; other than that, it didn't seem like an important issue.
However, I recently discovered at least one reason teachers ban first-person: prevented from using first-person, students will set aside me-centered thinking and use credible evidence; that is, rather than saying, "I think this, thus it is true," students will write, "According to expert X . . ."
I don't buy this argument; in fact, I think banning first-person usage ends up doing more damage than good. If the problem is the lack of expert/credible sources in students' writing, not using first-person doesn't solve the problem; it just covers it up. After all, a first-person's account could be more credible than an "expert's" account. I'd much rather read a student's personal/eyewitness account of 9/11 than a thousand third-person conspiracy theories.
The key is in the first sentence. Being forced to write in third person often results in stilted, boring prose. Unfortunately, the modern journalistic ethos, probably hammered into them in J-School, is that "objective" news stories must be written third person. This is why good bloggers (even taking away the bias) write far better and more readable pieces, than most conventional journalists. They don't have to do it with one "I" tied behind their back.
[Via her post on liberal fascism and Calvinism]
As a reader, material written from a first-person perspective is much easier to deal with. I can better understand where the writer is coming from and can put up the appropriate filters as needed. Just as the example of reading conspiracy theories alludes to.
As mentioned, writing third-person is often used as a strategy to maintain the pretense of unbiased material, which is far from the truth in most cases. The public would be much better served if journalists wrote "first-person" articles. However, that would make what they write just one persons opinion or perspective, which is exactly what it is. They wouldn't want that would they?
Other than "what I did on my Summer Vaction" type essays there is very little reason to write in the first person. The first person is however, far supperior to the detestible second person, which ought cause one to cease reading the piece imediatly.
The first person plural adopted by the scientific establishment, e.g. "We find that.." is a stylistic affection, and may be forgiven in that it is an expectation of the medium, but the third person reporting of the facts is clearly superior for presenting evidence or arguing a course of action.