Understandable, but at the same time, in order to defend a land mass the size of Canada against the US, we'd need an army bigger than China's. If the US wanted to annex Canada, no matter what kind of military force we had, we really wouldn't stand much of a chance - they have ten times our population, a far stronger and more diversified economy and a military system that's technologically superior to ours is almost every way. In addition to this, a great deal of our defence materials and equipment come from the US, so we would be reduced to fighting a guerrila war, which I don't know how long we would be able to sustain if the US controlled all shipping traffic in and out of North America. And as much as the rest of the world generally likes Canada, I don't know if any of them are friendly enough to go to war with the US for us. If the US was really bent on taking over Canada, there's pretty much nothing that we could do to stop them. Or even slow them down, really.
And since we have the US (the most economically and militarily powerful country in the world) as our neighbour, nobody's going to try to attack or invade us. Personally, I think that we should maintain enough of a force to keep our overseas commitments (which would imply either stepping up our military capacity, or cutting back on our overseas commitments - for the moment, the government seems to be taking the first road), while having enough back in Canada to respond to natural disasters, riots, etc. I would suggest that regular force be used for overseas deployments, due to the fact that they're much better prepared for such tasks (simply because they can train all day every day) than reserves, and reserves be trained and used during national emergencies where civilian agencies do not have the capacity to respond adequately.