I don't think they are really expecting to conficate things of tremendous magnitude during a bag check. We carried a backpack in January and they looked in it, but they certainly didn't root around in it. There could have been something small and dangerous concealed in the bag.
On the other hand, when we went to the Six Flags water park a few weeks ago we could have stashed a bazooka in our bag and they wouldn't have noticed. They allow outside food & drinks and people were wheeling in coolers big enough to put a dead body in and the guards were just glancing at whatever was on top. Good thing nobody was on a murderous rampage that day.
If somebody really wants to bring something dangerous into a park they will probably succeed. I think the bag checks might be a slight deterrent to people bringing in things that they really shouldn't, but as far as keeping us measurably safer I don't think it matters.