A United States of America comprised of the current country with the lands of Canada and Mexico could have advantages, although Mexico could have to develop a lot, get less corrupt, and increase per capita income. Canada in turn could have to get more sane and less liberal (and to some extent the same things as Mexico; Canada is not as developed as the current United States). The country would be the largest of the world's nations and would have lots of new resources, the largest amount of uranium of any country, lots of oil, lots of fresh water, lumber, diamonds, etc. Mexico also could provide a higher birth rate, thus keeping the United States as a developed country with a growing population.
No thanks. The trade off for free trade is too expensive. It sells out American sovereignty and buys into greater corruption.
As nicmarlo has already said, the price is too high. Anything with a negative impact on sovereignty, secure borders and culture, is a nonstarter and DOA with American conservatives.