This however, is perhaps reasons for their movement as a whole, not directly for the "flight 93" lie.
To some degree we are dealing with a tautology here. In order to be a "flight 93" conspiracy theorist, one has to contradict some portion of the official dogma of what happened on flight 93. The two most obvious points to attack are the presence/identity of the hijackers and the reason for the crash
Here is my list of reasons they make the claim:
1. Fits in with their world view and their view on the attacks over all (big brother did it)
2. One of the more plausible theories.
3. Hard to prove untrue.
4. Makes the pieces of the puzzle fit for them. They use pieces that are faulty to start with (the incorrect debris field covering 8-10 miles, etc...), so they must put forward a scenario that covers this, and a mid air shoot down seems to.
TAM
Here I think we are in general agreemnent: Taking these one by one:
1) agreed
2) I find none of the alternative theories particularly plausible, but since they begin with the premise in #1, "big brother did it", one can apply the Sherlock Holmes principle: once the impossible has been excluded, whatever is left, however plausible, must be the truth. Their problem is that once the truth has been discarded the subsequent reasoning is irrelevant. It's like a mathematical proof where you divide by 0 in step 1. No matter how flawless your reasoning in steps 2 through 20, you are going to arrive at the wrong answer.
3) Depends on your definition of proof. I think the preponderance of evidence, such as flight recorder data referred to above, effectively rules out a missile. However, since one can always claim that this was falsified, it will never be proven to their satisfaction no matter how much data you throw at them.
4) An inevitable problem when one starts from a faulty conclusion rather from the facts. It's like taking a jigsaw puzzle with a picture of a bird and deciding it is a picture of a house. If you take the blue feathers from the bird's breast and use them to make a sky, and the brown feathers from the tail and place them on the ground, and then use the tree branches to make the walls, and then throw out the rest of the pieces, and then place it in dim light, stand
really far away, and kinda squint your eyes, you can get something that sort of looks like a house. It may never stand up to close scrutiny, but if challenged you can always claim you are missing some of the pieces, or that even if it isn't a house it might still be an elephant, and then repeat the process.