"Nope - because oil is a world market"
Correct, except that it costs money to move. Having a continuous pipeline from Alberta to the Gulf Coast makes it dramatically cheaper to get the crude to the world market. Having the line terminate in Chicago makes it cheaper to refine and distribute regionally. This offsets shipping costs of bringing imported fuels in to the middle of the country. While oil as a whole is a fungible commodity in the concept of investment and pricing, the realistic implementation of it is still dependent on infrastructure and transportation.
"It will certainly reduce prices in the US by increasing the global oil supply."
The XL pipeline doesn't alter the world's supply. The same oil is already being pumped and refined, it just makes it cheaper to get to higher priced markets. It would reduce prices in the US if it were more profitable to sell in the US, which is largely what we currently see with the Keystone pipeline terminating in Chicago. With the termination point in the Gulf, the reduced cost of international distribution allows a greater profit to be earned by shipping it to other countries.
"Becoming a net exporter of oil would be terriffic"
And the XL pipeline would have no meaningful impact here. This is Canadian oil.
"and because we'd no longer have a strategic interest in the Middle-East "
The US doesn't currently have any strategic oil interests of our own in the Middle-East, and the XL pipeline would not impact that. The US only imports ~1/4 of our total oil consumption, the vast majority of that comes from Canada and Central America because it's closer and cheaper than floating barges over from Saudi Arabia.
Europe on the other hand, has extremely limited oil supplies, they are quite dependent on Russia, the eastern block states, and the Middle East for their fuel. And the XL pipeline, even with direct access to the coast, isn't going to push enough oil to offset any sort of major disruption from Saudi Arabia or Russia.
So in closing, no, the XL pipeline would not change us into a net exporter, it would not reduce gas prices in the US, and it would not have a meaningful impact on the global oil supply.