Personally, I think the attitude of the so-called "Evanghelical" atheists stems from the sheer frustration of proving P(1) logically so that P(1)and only P(1) is true, then having someone (a theist) deny it with no justification whatsoever and having to repeat the process again when another theist denies P(2) and so on until the argument becomes "pointless" as someone said above.

In layman's terms its like reasoning with your children over and over again that chewing on hard candy rots their teeth, and they still do it, until you get fed up and say "No candy, no questions." and then add a few mean words about how they dont listen to you.