The thread title is misleading and the posters have attempted to change the direction of the thread from obese to obese enough, but really, the original poster needs to provide clarification - the original post is vague enough such that each interpretation of the post is fairly loose. My argument still stands, though in regards to the people who are so obese that it is physically impossible for them to sit in one seat and therefore have a necessity to take up two, it becomes a bit more touchy. I'm of the opinion that if the people who are, what, 400-500 pounds are so adamantly foolish to only purchase one seat and therefore infringe upon the right that their neighboring passenger has to their seat, there should be some sort of reaction from the airline. Whether that be a rule or a flight attendant removing or relocating the passenger, I'm not completely sold.
Making a rule, or having to fill out your weight and height prior to boarding an airline, tends to be a difficult and widely unaccepted implementation. A person should not have to give out such specification to board an aircraft - whether a person be underweight or overweight, it causes problems. I'd like to think that most obese people know their limitations and are courteous enough to realize that if they are - for lack of a better term - spilling over into another seat, they would do something about it. I imagine this often results into them buying two seats rather than one, or flying in a higher class; either way, it costs more. In some cases - and my mother is a flight attendant, so I've heard many stories - some obese passengers will not move if they are taking up their neighbor's seat, which obviously results in a problem that needs solving.
I don't believe exercising a rule that requires people of a certain weight to purchase two seats would be the most elegant solution to this issue. There are, after all, different height to weight ratios that make all the difference in the world. Instead, I prefer a case by case basis solution - the flight attendants, more often than not, can handle these things, whether it be politely or by force. While these types of things happen, I can't imagine they're very prevalent in comparison to the many other issues airliners have to handle on a daily basis. I've only heard but a few stories of this sort from my mother, and I haven't seen them firsthand. Making a rule for an issue that is relatively uncommon would most likely do more harm than good.
If a morbidly obese person refuses to move, they deserve whatever is coming to them. This issue really boils down to one thing, and that is the individual in question. If they are 5'7 and 450 pounds, yeah, that's going to be a problem. It's not the right solution to implement a blanket rule that does not take into account the variance in ratio of height to weight.