That's true if you build the stupid corragated metal fences on the cheap that they are using now. Since you can't see through them, they just allow illegals to hide behind them until the BP passes. But if you build them right, they are quite effective. The Israeli fence along the Judea-Samaria border has been just about 100% effective in stopping terrorists from getting across.
I will concede that there may be some places where geography won't allow a fence (big canyons and such), but surely most of it could be.