These types of essays are prevalent. I think the reason is because a lot of the most powerful moments in our lives--the defining moments--are related to something that would generally be perceived as negative. The things is, I've found lots of applicants really need to write these essays. It's like exercising a demon or something. What a clever applicant does is write the essay--write it well, even--and then have the presence of mind not to send it to the admissions committees. "Pity" essays are funadementally written for the author, not the audience, and particularly not an audience one really, truly wants to impress.
I'd argue the main goals of the personal statement are to be remarkable, together, sound of mind, and most of all, likeable
. You want the adcomms to *like* you, right? Who wants to toss away the app of someone they like? Likeability is surely hard to achieve in a couple pages, but if a clever applicant keeps "likeability" in the back of their mind, I think they are more likely to pick a topic that will show them in a better light. (It's really amazing how much your present mood affects the writing you produce! There's a good tip--don't try and write your personal statement when you're in a funk.)
So--if you have a horrid story that you need to get out of your system, I say, write it! Just make sure that you write a couple different essays that aren't depressing, and then think hard about which essay will impress the adcomms the most.
I thought of two more 'common' essays that make my eyes roll:
1. The 'I will survive' essay - While I think it's perfectly acceptable to talk about circumstances that tested your perseverance and will to succeed, I find that most writers who choose this topic spend SO much time focusing solely on the negativity of their experiences that I only feel pity for them. They usually end the essay by saying that they're not a quitter or some other trite ending.
2. The 'Pity' essay - If the above topic generated self-pity, this essay generates pity for someone else. Whether it's a small impoverished boy, Brazilian prostitute, imprisoned orphan or whatever, these essays focus so much on the negativity (and usual recovery) of the subject that focus shifts drastically away from the writer. Essentially, it suffers the same problems as #1.