Not to mention that we have the longest undefended border in the world between us. Treating Canada poorly 'could', I suppose, change that and that (along with them being our best trade partner) would truly be a shame.
I read recently that Canada has ramped up their defense spending, and although I'm happy to see that, since I believe it's in our (the U.S.) best interest, it will be nowhere near what it was years ago.
Their creeping socialism, national health care and Eurostyle attitudes make it necessary that they stretch their limited defense dollars and IMO it's best spent on conventional 'everyday' hardware to benefit their troops.
I don't think we really need their help to build the ABM. Just as long as Canadians are aware it might not defend Canada all that well.