I think ballistic missiles have two functions that will matter. The first is for an out-and-out attack. We want to prevent that. It seems unlikely short of a state of war, that Russia or China, being relatively responsible states, are going to launch a missile intending to hit a Canadian city. We don't know about the sort of more unreliable governments like North Korea's, and even to some extent Iran's, and because those governments aren't entirely trustworthy, we do have to worry about what the risk is of them taking a strike.
The second issue is the threat. I think that ballistic missiles and long-range cruise missiles, as Professor Doran highlighted, have the ability to be a threat, and to put people in a vulnerable position in negotiations, etc. It hangs over a population and creates a sense of anxiety. It may be that Canada's investment in missile defence is circumscribed. It's relatively limited. The advantage you have is the systems have already been developed. There are things you can buy off the shelf now that had you done this in 2005, or earlier, you might have had to invest in R and D and develop the systems. Now you're in a position to be able to make a relatively modest investment to deter threats. I don't think it has to displace the need to recapitalize the Canadian navy or purchase fighters, but I think a modest investment here would pay good dividends.