I'm not disregarding it. I'm suggesting we should be working towards formulating economics, politics and everything else which works out for a borderless planet.
In regards to why I advocate it - like I said, the way the system works today, some people have more privilege than the others, and this isn't by choice. We should be trying to bring everyone upto the same datum, not the other way around. While the current visa systems are not the texbook definition of discrimination, we are in some way still limiting people's choices based on the land they were born in.
If we're talking first principles, a human should be able to roam wherever he chooses. Of course, this isn't an ideal world, and I'm not advocating for terrorists to be able to make it through wherever they wish, but I'm sure if thought hard enough, we'll be able to come up with a system that ensures that a couple of groups of terrorist, and a few other kinds of knobheads don't pull their entire nations down. But right now, we're working in the opposite direction.