I think fencing in strategic populated areas does slow down immigrants enough, 5 to 10 minutes or so to climb over it, for border patrol to catch those crossing....
My problem is with a wall or now being changed to fence, in the middle of the dessert, where no roads or city or town for the illegals to go to nearby once they cross, is an absolute waste of money...mainly because of the rarity of crossing and because slowing them down with a fence, does no good if border patrol is not within a few minutes of them crossing, so they can be caught by them...and increased spending on border guards and high tech systems in areas that attract those trying to cross etc, would be money better spent.