Some private (corporate) jets are the same size as typical commercial aircraft.
We learned on 9/11 that as soon as word got out about the possibility of terrorists using aircraft as a weapon, the passengers are ready to tackle anyone who appears to be doing this, so the threat was already gone months before the TSA was formed.
Similarly, the military would now shoot down an aircraft in similar circumstances. This does not address the loss-of-life concerns, but does redundantly prevent the use of an aircraft as a weapon.
Also, since it is easy to get weapons through TSA security (it's done often by other government agencies as part of various audits)... There is no reason to believe that the TSA line makes anyone any safer (though the above points do make people safer).
And, as Bruce Schneier points out, one could simply put explosive liquid in a tomato sauce container and attempt to go through security. If it's found, it would be thrown away and the passenger would board without being caught. If it's missed, the system has been thwarted and an attack can occur.
So aside from the initials in TSA reminding of us the concept of security, the agency does little to actually make anyone safer. I'd argue that we have a false sense of security against all but the most unsophisticated attacker.
Also, the main reason the TSA was created was to shield private firms from risk if a banned item got through security and did result in an attack. There was a big restructuring of the terrorism insurance market post 9/11, and the government now "owns" much of the risk. This was thought to be necessary to keep planes flying after 9/11, since the market price of insurance (and flights) would go through the roof if an airline would be held accountable for a 9/11 style attack if one of its minimum wage employees missed a box cutter in someone's bag.
So now we have the TSA and an absurdly regulated public/private partnership that fails in every measurable way and as others have pointed out wastes a tremendous amount of time and resources.
I apologize for the rant, I'm sure you have heard this argument before.
