They're damned if they do and damned if they don't. No "threats," people will just see them as a nuisance. Something happens and everyone is happy they're there. I don't work for Dis nor do I work for any security detail so I don't know exactly all that goes into Disney security program but security is now a part of our lives whether we want it, need it, or like.
Keep in mind, it's not like Disney reports to you directly everytime they stop a threat nor do they share with you every memo they receive from law enforcement at the state and federal level about possible threats. Security doesn't also just stop terrorists threats. They're there to stop protests or homegrown "terrorist" threats. People seem to think that terrorist is only about a foreigner "attacking" our country. We've had some large scale attacks over the years from foreign regimes but we've also had mentally ill people that are "american" carry out attacks too. School shootings, church shooting, mall shootings, etc...
I'm not saying they're going to "catch" any and all situations such as the pocket knife example that some shared earlier nor do I know if that is their number 1 priority but having them there is better even if they stopped one instance.
When I go to disneyland, it's always with my family so I don't want to be in any situation where my family is enjoying a trip to Disneyland that is then compromised due to any kind of dangerous activity. It sucks to go through "security." It sucks to be treated like you're guilty or assumed that you're bringing something into Disneyland when you shouldn't but I'd rather have them TRY and discover and prevent any bad situations then do absolutely nothing at all. My family is worth that extra "hassle."