I understand what you are saying, but now that both the Bakken & Canadian oil are ramping up, it would seem reasonable for the oil companies to be pushing for a refinery rather than a pipeline.
Refineries need two things:
1. A pipeline network to bring crude oil to them.
2. A separate pipeline network to distribute finished products to the market.
The Gulf Coast refineries already possess the latter. A hypothetical refinery in Montana possesses neither.
Moreover, the economics of refining dictate that a refinery be run as close as possible to 100% of capacity. Shipping the 800,000 bpd from Alberta to the Gulf Coast will immediately displace 800,000 bpd of other imported oil -- chiefly from the likes of Venezuela -- improvng our independence from foreign sources.
By the way, it is not just Gulf Coast refineries that would benefit from the project. There is already a leg of the Keystone pipeline in place that delivers to refineries in the Midwest (St. Louis/Wood River complex).