[teyc]

They are impossible to fire.

If no one slips a bomb past them, they've done their job; If someone slips a bomb past them, they'll only grow bigger and more menacing.

[sophacles]

Worse still: If no one tries to slip a bomb past them, they must ignore the obvious and declare they need more power to catch the evil-doers. If someone is successfully caught by them, we obviously need to up security, because this display of effectiveness may not be enough.