I've seen a lot of people trying to find a good analogy for the situation of mages in Thedas, and it seems to be pretty hard.I think a pretty good analogy could be real world people with serious mental illnesses. In the real world, such people are normally kept under some form of supervision, even though they might be quite sane for the most part. They might seem like normal people, but then suddenly lose touch with reality and hurt themselves or other people.
In the past, and not too long ago, most people with delusions were considered lunatics and were put into asylums for the rest of their lives. This was, of course, because they were considered a hazard to society. This would probably be because society didn't really have the resources to deal with them. We're normally a bit on the judgemental side so crazy people isn't normally up our alley.
Some people may be aware of their illness and recognize their symptoms. We normally let these live in their own homes, let them do a bit of medication and make sure someone keeps an eye on them. Those that can't do that are given more drugs which normally leads them into a state that could be compared to the Tranquil (not quite as bad, but still). If someone can't be controlled by medication, we lock them up.
When someone does something horrible, like the recent Arizona shooting, one of the first things asked is often why society didn't see this coming. Why wasn't this person supervised, considering how unstable he apparently was. As soon as something happens, apparently people like to see more control and less freedom.
Mages are of course rational beings, but it has to be said that once they've been possessed, the mage is kind of out of the game. It doesn't happen voluntarily, so it's not as if it can be prevented if the mages just wanted it a bit more. For the people around them, it must be pretty uncomfortable to live with the constant risk. When someone loses touch with reality in the real world, anything could happen, I'm guessing that for the most part someone just sees things that isn't there. In Thedas, it's pretty likely to be a killing spree. If that had been the case in the real world, I'm guessing that we would have things quite similar to the Circle, no matter the love for freedom.
I'm sure this one has been mentioned by someone else in some other thread, but I just thought of it myself.