I think the TSA has switched strategies recently, and they've been improving since then. But ultimately, its not the "bag check" where the TSA catches goons, its the random more intense check that they do these days. And sure, going from 95% failure to 80% failure isn't much of an improvement, but its still going to change the calculus of an attacker. When you have a 20% chance of failure before you even board the airplane, that's an issue to your plans.
Furthermore, TSA includes air marshals and the Israeli-style "behavioral" cops who are scanning airports. These are the more successful cops at the job, and the ones we are relying upon for real.
I wouldn't discount the entirety of the TSA just because of the failures of the bag-check program. Again, US Air marshals (armed cops who sit on every aircraft secretly with plain-clothes) are under the TSA and I think the US Air Marshals contributes a LOT to our real security.
Unfortunately, the bag-check thing is mostly security theater, but at the same time, the more "serious" checks vs attackers are more hidden and less well known (as it should be). But security theater might be enough to discourage an attack, or at very least cause our enemies to focus on the bag-check (when our REAL security really lies with other programs). So its a useful distraction IMO. There's a bit of "wink-wink lets not talk about this" going on that you need to understand though.