Canadian customs are unbelievably incompetent, so an open border wouldn't be a good thing.
They don't even currently search you while entering (as long as you act cool, present a passport, and maybe a verifiable itinerary such as a hotel reservation), so I don't really see what difference it would make. The main benefit would be for Americans returning from Canada, not having to deal with idiotic and dickish customs agents. Say what you will about opebo, but he is 100% correct on his characterization of US customs officials.
And as Ernest mentioned, Canadian weed only accounts for like 5% of the US supply, and most "BC Bud" is probably located around the Seattle metro area. This leads to highly inflated marijuana prices throughout the Midwest USA (sometimes 2x expensive than EU levels and 1.5x more expensive than Southern Ontario). Ideally open borders would facilitate a drop in prices to like $45 per 3.5 grams in cities like Chicago, Minneapolis, Milwaukee, and various Big Ten college towns.
