That's not quite right. Very few people are found carrying guns through metal-detectors, but this doesn't mean that metal detectors have no place in stopping guns from planes.

The step that screens liquids isn't going to actually find explosive liquids; it is going to stop all liquids, so the bombers won't be bringing them through in the first place.

I can hear lots of good arguments that the restrictions on liquids doesn't pass a cost-benefit analysis, but once you make the decision to stop liquids, having a common disposal place really isn't the problem. Someone could just suicide-bomb the security line in the first place.