Well they are an illegal association, so not trusting them seems wise. But the question here is not whether they're outlaws or not, since they clearly are. What I question is the Chantry and Templars statement that left to themselves, mages are bound to bring great destruction, that they are so powerful they cannot be contained. I'm just observing that somehow outlaw mages are managing to not set people randomly on fire, to not become spontaneous abominations, to not kill tens of people. And this is happening in an illegal, unsupervised environment. They do commit crimes, but they are no more horrible than their mundane counterpart.
We have no concrete numbers both on the members of the Collective or that of how many magical accidents are caused, precisely because of their actions. Thus, we can only have a perception of them by their methods.
Some of the requests are highly suspicious and their "policing system" relies entirely on luck.
For instance, there is a quest where we are supposed to check on an apprentice that has failed to contact its master. When we reach him, he's an Abomination.
Has that Abomination killed already? If it hasn't, then that was only possible through tremendous luck. What if no one had take the request? What if someone less skilled than the Warden did and was killed?
Had that apprentice been in a Circle, none of that would have happened. Even if he became possessed, he would be surrounded by an army of Templars and away from potential victims.