Here are my thoughts on security along the Mexican border. I've been mulling it over and began to look at it as a military objective. The thing I look at on putting up a fence is it's far too easy to circumvent. You will still have the Atlantic, Pacific, and Gulf coasts to secure which isn't being suggested. If someone wants to get in they'll still be able to. In fact you give the ones trying to get in a bit of time and they'll soon have a tunnel underneath dug.
That leaves some sort of patrolling. I recommend taking a bunch of those drones, interface them with INS, and of course hire a bunch of the ones who were piloting them over Afghanistan and Iraq. The drones can run the entire border pretty efficiently including both coasts.
During WWII the Maginot line was considered the best possible secured border. What did the German military machine do? They simply bypassed it and went on into Belgium and France.
Going back a further in time to the occupation of Britain I call attention to Hadrian's Wall. The first one wasn't very effective and there were a lot of incursions. A second wall was built as a result about twenty miles south of the first one. It also managed to get a lot of incursions. In the end we all know the Roman Empire left Britain.
I can pull up many more instances like this but these are two of the most famous ones.
By putting up a fence along the southern border you're asking for the same kind of thing. While I understand the need it needs to be better thought out than seems to be at present.