The price of crude oil and refined products is (mostly) set in the world market. Therefore increased supplies of product will result in lower prices, regardless of whether any of the molecules from Alberta actually find their way into the US market.There is a reason for it that most people never even thought of: US citizens most likely will not see a drop of that oil; it will most likely be shipped via a barge to another country so that other country can use that oil.
(I don't support the pipeline, but I'd prefer that arguments against it are sound.)
The argument against the pipeline that I think ought to have the greatest traction in the current American political scene is this: when (not if) the Keystone XL pipeline leaks, it will be American soil and aquifers that are contaminated: we get the environmental risk, while the Albertan tar-sands producers and the pipeline owners get the profits. If things go really bad, it's American lives at stake fighting fires, etc. As we're seeing with BP, don't expect that the company responsible for the accident will make everything right and everyone whole.
The real argument ought to be about carbon and the high environmental cost of exploiting the Albertan tar sands, but that's not going to carry the day in current American politics.