How does exporting oil make me safe at night?
Christ almighty...
Okay, so if the Keystone was to be built, over the objections of the "I love the spotted jumping western field mouse" envio-nazi crowd of course, it creates a steady flow of oil into this nation. Now yes, the plans are for it to go OUT in exports, but it still has to come IN. Anyway, let's say the ME gets into a tiff (again) and does another embargo (i.e. 1973) we would be screwed without production to take up the slack.
Enter Keystone that can be turned off for exports, as Hog said, and kept for the domestic market. So our reliance on overseas oil isn't as big a deal if, IF we have the capability to take up some of the slack.
So while we are still reliant on overseas oil, we would have the potential to get by without it by shutting off our exports. Hence, national security has a status quo as power grids stay up, vehicles still run, aircraft still fly and life goes on.
And not even getting into the aspect if China gets their way with loading it up in Vancouver and sending it to their refineries.