That is the question.
We've all heard it spoken enough that to repeat it invites a gag reflex; that immigrants come to Canada to contribute to it. As noble as this sounds I think this is hardly the case given human nature.
To contribute to implies an act of selflessness as if Canada and Canadians are in desperate need of outside help. I wouldn't describe Canada as a struggling nation nor Canadians as a struggling people so what exactly are we importing people to contribute to? Besides, if selfless contribution is at the heart of every immigrant to Canada then there are plenty on nations around the globe that could use their help more so than Canada. However, I do understand that it is more fulfilling to help a rich person who may also make you rich in the process.
To take advantage of implies a selfish act for personal gain and this to me is the more accurate motive bringing many people to Canada's shores. And I say that not to disparage those who come here but an acknowledgement of the human impulses that compels someone to act one way or another. Self-interest is stronger than altruism.
The linked Vdare article above makes that case. Though it is about the United States it is equally applied to Canada. And though it is about Chinese immigrants the behaviour outlined is not restricted to them.
The motives driving immigrants to Canada is important because it is an indication of what kind of citizens they will be and what Canada really means to them. If Canada is nothing more to them than a shopping mall, a job, an urban sprawl home, and a list of social benefits; health care, education, passport; then the desire to assimilate and be "more Canadian than Canadians" is elusive if at all present. This desire is further hindered by the current state of communications technology and affordable travel which allows one to live a satellite existence in Canada away from the home country. The ever growing foreign presence in Canada, colonies euphemistically referred to as "communities" or "enclaves", let's one live in Canada without actually leaving the homeland.
If contributing to Canada was the real intent then "being Canadian more than Canadians" would be desired but this is no longer the case like it once was. What we experience instead are incessant demands for accommodation which is the demand on the host culture to reshape itself and rethink itself so the introduced one doesn't have to. What we have are so-called "Canadians of convenience", a greater phenomenon than we care to accept.
The way to screen out the opportunists is to make it more difficult for them. This would require rewriting the citizenship laws denying birthright citizenship to anyone born on Canadian territory (or airspace as the case may be) to non-citizen parents. Another would be to rethink dual citizenship. Another would be to make residency requirements tougher, possibly with routine reviews to see if the requirements are being met and a penalty of forfeiture of citizenship if they are not. Another would be to close the loopholes one can exploit to dodge Canadian taxes while drawing an overseas income. These are just a few ideas.
Making it tougher for immigrants to come to Canada and become Canadian citizens would attract those who sincerely desire to come to Canada and repel those who see Canadian citizenship simply as the best deal in town. Something worked for is better appreciated than something not and by making immigrants work for their citizenship will make better citizens of them. Right now, this is something we don't do. Canada is more than just a land of benefits and entitlements.