Akri said:
For a sport (many sports carry a risk of causing injury to one another, such as boxing), for sexual pleasure (masochism), to impress a chick (I believe Dinwar has a story on that--I'll leave it to him to decide if he wants to share), and probably other reasons I'm not thinking of.
I've got several.

Most of the SCA is honestly about impressing chicks, and I've got a deformed shoulder thanks to that. Then there was that time I let a girl cut my arm because she wanted to see blood. Honestly, that's the story--she said "I want to see blood." She was hot, and I'd just gotten a black eye from practicing fencing with Gateraid bottles (I at least emptied mine first), and thought why not? It's not like I don't bleed on a regular basis anyway (no genetic disorder, my family simply doesn't consider bleeding to be worth worrying about). So I let her cut my arm.
Consent to be harmed goes beyond that, however. There's also assumed risk. Doing things like swordplay or hiking in mountains in deserts is inherently hazardous. By doing those things I consent to taking those risks--meaning that if I get harmed, I have no one to blame but myself. This is not hypothetical, by the way--when I got my shoulder busted in the SCA they asked if I was going to press charges, and I honestly had to ask "Against who?" I knew the risk, and did it anyway. I may not have been the one that hit my shoulder to make it break, but I was just as at fault as I would have been if I were alone and did it to myself.
The issue for a rational person is that one must look at the potential benefits, and the potential costs, and see if one can afford it. The benefits of fighting in the SCA are well worth a few medical bills. Seeing that girl naked was well worth the inconsequential blood loss (honestly, I've lost more making my bed than she drew). The benefits from hiking in the desolate mountains are worth the occasional moment of sheer, unbridled terror.
What you CANNOT do is violate anyone's rights. That is an absolute that cannot be circumvented by pragmatic post-hoc justifications.
Earthborn said:
If so, doesn't that mean the "harm" done to this person is actually something they value and therefore no harm at all?
No, of course not. Sometimes, yes--in the case of a sexual masochist, for example. In other cases, no--it's merely a potential natural consequence of the action, and has no greater significance.
If not, does Objectivism allow people to take advantage of the "irrationality" or "non-self-interestedness" of others?
It doesn't forbid it, but all O'ist philosophers (yes, there are multiple; published in peer-reviewed journals and everything!) agree that doing so intentionally is self-destructive. You're basically rewarding irrationality, which will necessarily harm you in the long run. Now, this does NOT mean that we all agree on what actions ARE rational or in our self-interest; it merely means that intentionally pandering to irrationality is self-destructive. There is a difference between an error of knowledge and a moral failing.
Is it moral for a Randian hero get rich by exploiting the irrationality and willingness of self-sacrifice of others, despite disapproving of those things?
Dr. Simon Pritchet illustrates the flaw in that reasoning. So, for that matter, did the former chair of the Fed.
MontagK505 said:
Obviously, we start skirting around the "No True Scotsman Fallacy" here.
First, O'ism is a philosophy; while there may be some that hold to it irrationally, to term us "True Believers" is a misnomer and slanderous. Many of us believe this philosophy for entirely rational reasons--same as you people believe in skepticism for what you hold to be rational reasons.
Second, no, we're not. If you violate fundamental principles of a philosophy it is no fallacy to say you're not following that philosophy. To say "You're no true Scotsman" to a man who's never been to Scotland and who is in fact an American citizen is not a fallacy.