My frustration is when people say one thing and then immediately undermine it with a disclaimer or a major exception. The classic example is those "I'm not a racist, but..." remarks, but the one that set me off just now is "I consider myself pretty liberal, but I'm no tree hugger." Using conservative phrasing to oversimplify and vilify one of the major areas of liberal concern (environmentalism and conservation) is not exactly a good way to establish liberal bona fides. I bet you a steak dinner that this person is nominally liberal but has never actually given much thought to it.
Come now, I doubt you'd be this critical to someone's bona fides if someone like me were to say "I'm a Christian, but I'm not a Bible thumper." Proselytizing is just as important to the majority of Christian attitudes as environmentalism is to liberals, but it is possible to be a Christian or liberal and have a certain disdain with a particular practice or belief of it.
The built in bias English has when dealing with mental disorders, and the culture that bias reflects. A person has the flu; a person is bipolar. Stigmatizing a health issue is foolish and cruel, particularly when one can do nothing to influence whether or not they poses it.
RD, you're referring to two different things in this case. Bi-polar as you used it is an adjective, and flu is a noun. You don't use the possessive with an adjective, nor do you use "to be" with a noun(very rarely, anyways). It's not uncommon to hear someone say "He has bi-polar disorder," over "He is bi-polar." The inverse is also true. A person can be feverish, or they can have a fever.
But at this point I'm probably just trying to be difficult. I see what you're getting at here, and yeah, the scale seems to be unfavorably tipped towards mental disorders. Perhaps it has to do with the nature of the disease, and that mental disorders change the person's actual being. I doubt we would disagree that bi-polar disorder affects one's personality and individual being more than the flu does.