You depend on us for your defense. Why would you even bother! DUH! If my country had such an arrangement, I'm sure we would choose impotence too~!
I am the first one to admit that Canada DOES depend on the United States for its defense, and we have for as long as I've been around and many years before that as well. Canadians would be deluding themselves to suggest otherwise. (If any Canadians on here doubt that, read about the history of NORAD). It's called an "alliance", and for our part we are 1/10th the size (ie-population) of the United States, so of course the alliance will be an uneven partnership.
On our side, Canada has long benefited by the fact that we are strategically protect by a nuclear and conventional deterrent that has, let's be honest, been paid for by American taxpayers. We have a small conventional force as well but, with all due respect to our own Canadian forces, it is minute in comparison with American forces. (If I'm not mistaken, ONE American aircraft carrier has more tonnage than the entire Canadian navy... or at least pretty close to that!)
However, I would suggest that the American side benefits as well. Strategically, for as long as living memory can attest, America has not had the slightest strategic worry of a military force threat from its long northern land border. Canada (sorry to be so blunt, Canadians!) is the world's largest military buffer zone! I would suggest that Canada being completely peaceful with respect to the United States is a very good strategic thing for the U.S. Take a look at where Southern Ontario is with respect to the United States. It dips down awfully deeply into the middle of the American area of the continent (the southern tip of Ontario is at almost the same latitude as the northern tip of California). A hostile force in Southern Ontario could never be strategically accepted by the United States... WAY too close to New York, Washington and Chicago, etc. Canada's solid military alliance with the United States means the United States gets all the strategic value of the 'buffer' effect, without ever having to go to the expense, effort, and cost in blood of a war and long-term occupation of a neighbor.
Furthermore, in terms of the unevenness of the expenditure aspect, let me suggest that the United States is just as happy with it that way. Suppose, hypothetically, Canada decided it wanted to become a nuclear weapons power. How would the Pentagon feel about that? Even if we said it was "strictly to the benefit of doing our part in the alliance", I don't for a minute believe that U.S. strategists would see a foreign nuclear missile force based half-way between Milwaukee and Boston as a good thing (and yes, draw a line between Milwaukee and Boston, and the midpoint is in Canadian territory!) Instead, how about a massive land mass that is completely allied militarily with the U.S. (ie- would not permit Chinese or Russian military forces on its territory)? Zero military threat, fairly consistent military cooperation, no need think about potential threats. Strategically, that must have some appeal.
From a geo-strategic point of view, Canadian territory IS effectively American territory, and that's long been the case (I would suggest that the Manhattan Project was one of the most important U.S. strategic projects of the 20th century... two of the project's sites (Chalk River and Trail) were on Canadian territory). But again, I think that's mutually-beneficial, and neither nation has any real geopolitical interest in changing that fact.
I've said I absolutely oppose Canada's constitutional relationship with Britain, but I have no problem with our military relationship with the U.S. You might refer to it as "impotence". I call it "alliance". And one that both sides seem happy to maintain.