Ron Paul's district is Freeport, Tx - and isn't sitting 'on the border'. He might have a different view if he had lived in Laredo- which is right on the border. The distance from Freeport to Laredo (about the closest point of the border to Freeport) is 307 miles. The same distance that Boston is from Montreal.
Yes, those other factors listed are important. However, a wall WILL be effective, very effective, in many places. It won't stop it all- but it will stop a hell of a lot. Just ask Israel ... and China. *s*

Finally, brethren, whatsoever things are true, whatsoever things are honest, whatsoever things are just, whatsoever things are pure, whatsoever things are lovely, whatsoever things are of good ...