If you're talking about dual intention, it's not exactly that it's not correctly applied; it's that they are very miserly with its application. You can't say that there's anything legally wrong, it's just that they're so suspicious and the tendency is to just refuse and refuse. I was saying earlier that one of my colleagues called it the “kiss of death” if the person you're coming to visit in Canada happens to be your spouse or common-law partner.
Systemically, I think there are some issues too. People who are visa exempt for Canada, who when they come to the border have exactly the same legal test as everyone else, seem to have a much easier go of convincing an officer of dual intention; whereas a person from a visa-required country has a much more difficult time persuading an officer that they have a genuine intention to go back home after the visit.