In fact, I think that if you have two braziers that can fire on a location, and add a torchpost that can also hit there, the damage is unchanged.
Compare (3+3)/(1.2) to (3+3+1)/1.4, and you will see that both equal 5.
For a single Brazier, a Torchpost will give a minor assist. (3)/(1.0) vs. (3+1)/(1.2) = 3 vs 3.33
For two Braziers, torchposts add NOTHING (as shown above). Place as many as you want, but the damage they deal is only enough to offset ho much they penalized the Braziers.
For THREE Braziers, a torchpost will actually HURT the total damage output: (3+3+3)/(1.4) = 6.43; (3+3+3+1)/(1.6) = 6.25; (3+3+3+1+1)/(1.8) = 6.11
Since Torchposts are weaksauce, provide NEGATIVE damage when coupled with mass Braziers, and Iron can be mass-produced in ways that Leather cannot (due to the limited brain supply), it seems that there is no reason to ever use torchposts as defensive measures.
...All of this assumes of course that the equation on the Wiki is correct. Which it surely must be!