If national governments don't have any rules, and they all do—sometimes enforced properly, sometimes not—the existence of the Arms Trade Treaty is not going to have any effect on that whatsoever, because the treaty reflects consensus behaviour.
In my view, these obligations are interesting and they are driven by other political considerations. In the case of Canada, if we stick it in the context of Canadian foreign policy, multilateralism, we sign on to something that really has no clear implications for a country like Canada, which always has rigorous export controls. That there will always be problems and that there will always be cheating, from either an internal or an external perspective, is nothing that will be resolved by an international treaty.
I know I'm a bit extreme and a bit of a Canadian heretic, but that's me. Signing the treaties makes us feel good, but they don't do much more than that, at the end of the day.