I have long been fascinated by the way in which certain words become loaded with positive or negative meanings and, as a result, become used in some truly bizarre ways. This has been the fate of the word rational which has grown beyond being simply descriptive of an analytical process and taken on a rather strange life of its own.
Rationality is essentially the process of careful, logical, and honest assesement based on available information. Rationality is wonderful for determining the validity of empirical claims. However, not even the most die-hard self-proclaimed rationalist would claim that rationality and logic are all that there is to life. We are human, after all, and emotion - the very opposite of rationality - is vital to our well-being, and sometimes we have to chuck rationality out the window to find happiness. Listening to beautiful music, finding a poem moving, becoming involved in a well-made film, becoming invested in characters from a novel, and falling in love are all emotional, non-rational things, and our lives would be poorer without them.
However, that is not to say that rationality should play no rule in even emotional decision making. After all, we can look at our own histories and our desires and use logic to reach some conclusions as to which courses of action are likely to be beneficial to our long-term emotional state, helping us to see what makes us happy and what makes us miserable. And no matter what your heart may tell you right now, your long-term happiness is likely to be compromised if you marry that meth-addicted convict whose favorite hobby is spreading herpes to Chihuahuas - that's a rational assessment, and you can trust me on this one.
Rationality is one of many types of mental processes, a vital and important one to be certain, but not the only one. It is absolutely necessary in assessing the truth of empirical claims (and as such is vital in evaluating things ranging from a salesman's pitch to public policy proposals), and can of great help in our decision making, but it is simply a descriptive term for one type of mental process.
However, the term has come to take a broader meaning. Rationality has become a term generally associated with positive mental health and clear thinking. When we say that someone is irrational, it usually doesn't mean that they are being emotional or asking non-empirical questions - it usually is used as a pejorative meaning that the target is not quite sane, or not to be trusted.
Let's be clear, I don't necessarilly see this elevation of rationalism as a problem. While it is only one mental process and others are necessary, it is one in which we as a species tend to be rather unfortunately deficient. If we were encouraged to actually be more rational then that would be wonderful.
Unfortunately, that isn't usually what happens. Instead, most of us find ways to justify our irrational positions and try to claim them as rational. So, we find people coming up with rather tortured explanations of why racial group X is "clearly" biologically inferior, why assuming without evidence that something is true (AKA faith) is "actually quite rational", or why some bizarre and unreasonable conclusion is really the truth. This isn't rationality, but rather it is the spinning of rationalizations - rationality's bastard mutant cousin who lives in the attic and enjoys humping spiders.
Often such rationalizations are inconsequential - rationalizing why Humphrey Bogart was the greatest actor ever to have lived, or coming up with an excuse for believing that unicorns kick ass isn't going to hurt anybody. However, when one becomes accustomed to rationaliztion their pre-existing assumptions about race, politics, sexual orientation, religion, etc., then this provides a place for people to dig in and advance positions that are in fact harmful.
In a way, I wonder if we wouldn't be better off if rationality weren't as highly prized, then we could point out assumptions without those holding the assumptions trying to rationalize them. Of course, then, there would also be no reason for the person to change their position when proven wrong, so perhaps we'd simply be in the same boat.