We aren't talking about the "rare" individuals. The obesity problem seen in the United States is almost exclusively due to people eating crap food (sugar sugar sugar) and not exercising, period.

I am not a doctor, but I'd still be willing to wager that the health benefits of going on a diet/exercise regimen, thus losing weight, FAR outweigh any potential downsides to dieting. Seriously, how can you say "oh, overweight people might be harmed by diet restrictions and exercise" when for 99% of the cases out there this simply isn't the case?

There is no way to lose weight besides caloric intake being less than caloric expenditure. By saying "you are making yourself weaker and more vulnerable to disease" you are effectively saying (though probably not meaning) "you should not slim down". Trying to drop 25 pounds in a week is pretty insane, but a well balanced 1-2 year plan, approved by a doctor, will go a long ways towards health and not against it.