I seriously believe that where you appear weak (our border enforcement) your enemy will ultimately exploit it. Besides, it's an image 'thingy' and it would be wise if we did our best to uphold it. The world's last great superpower ought not have an exposed underbelly. Sneaking into this country ought not be any easier than sneaking onto a Marine base.
But border patrol has never been a priority in this country, yet the US has won many a wars. I understand your point, but I don't think the rest of the world perceives Mexico as an enemy of America. Hell, you might as well include the Canadians - we're all Americans.