How challenging would it be to poison a reservoir, poison random groceries across the country, blow up the Alaska Pipeline, etc. Any motivated terrorist would attack us where we aren't so vigilant. I'm honestly pretty unsure about how many motivated terrorists there are out there-- if there were out there (and well-financed), couldn't they be crippling us with a dozen different types of attacks?
The TSA is like putting your hand on a screen door to keep out the wind. We've got better things to do with our time and money.