Currently the United States has installed two land-based systems already in North America. There are systems being developed for Europe. The most effective systems are Aegis missile defence destroyers.
Within that current construct, what concerns me is to what degree, as part of the planning and implementation of the system, if we do see a ballistic missile accidental launch or anything else that comes towards North America, there is active consideration of the trade-offs that might be made in terms of how you launch to protect as many cities as possible.
We simply don't know if the United States, at this point, would be willing to sacrifice certain defensive launches in order to maximize the protection of American cities, or if they would spread it in such a way that any potential Canadian cities would also be protected. Whether or not we choose to join the system ultimately, it would be good for us to at least have an understanding of what the planning is at that level so that we can truly say that in the aerospace realm, where we already provide data to a system that's meant to protect North America, we are an active participant in it.
Again, we endorsed this for Europe and for the United States, and it seems odd, to my mind at the very least, that if we were willing to endorse it for our allies we wouldn't be willing to at least consider it for ourselves.