"We defend this city, not to protect these stones, but the people living within these walls."
--Balian of Ibelin (Kingdom of Heaven)
A common argument against Trump's Wall is that walls do not keep people from crossing borders. People can climb them, go around them, etc.
An underlying assumption in this train of thought is that all border crossers are equally talented and motivated. But they are not.
Similar to locks, safes, et al,, walls deter crime--particularly crimes committed by 'low level' criminals. When we lock our car doors, for instance, we deter less sophisticated thieves who possess neither the know-how, patience, and/or desire to break the security system.
Even when criminals are 'enthusiastic' in nature, deterrents buy time for potential victims. The longer it takes to break the lock, crack the safe, or scale the wall, the more likely that the criminal gets caught. That prospect alone might turn away capable criminals. If perps decide to proceed, then they realize that they are on the clock and exposed to detection.
Survey questions to those who oppose Trump's Wall. 1) Do you lock your car doors when leaving your vehicle unattended? 2) Do you lock your home at night? 3) Do you have a exterior fence or wall on your property? 4) Why do you do 1), 2), or 3) when you know that there are individuals out there who are capable of breaking your security?
There may be other reasons why Trump's Wall is truly a bad idea. But arguing that it will not serve as a deterrent to illegal border crossings is a red herring.