The problem with the latter scenario is you are dumped into a customs hall at a U.S. airport with dozens of other flights from Latin America, the Caribbean and Europe and end up waiting forever. Part of the idea of clearing in Canada is to avoid the gong show upon arrival at your first U.S. airport. Yes, preclearance facilities can be busy but they don't suffer from the "widebody dump" of passengers.

The same problem used to exist at Heathrow before Canadians and other Western countries were allowed to use the e-gates. Your flight from Canada would hit the immigration hall right around the same time as widebody flights from India and Africa, the hall would be swamped and you'd spend easily an hour in a slow-moving line waiting to see an officer.

One of the few things I like about flying AC from the U.S. or Europe and connecting at YYZ or YVR is that I can use the domestic connections facility upstairs, which again has shorter lines and avoids the widebody dump into the main hall.