1) You are correct, it is a disabling mod.
2) I am not throwing in an extra battery. The deep cycle has replaced my starting battery.
3) This mod is extremely easy to undo as Eggman has already posted. To completely undo it you simply unplug the circuit and replace the fuse.
With current fuel prices (used $2.85 as that is the national average) this mod it will save ~$100 in the next 17k miles. In 100k miles it will save me around $600. If gas goes up, I save more. In SteveDMC's case, he would have saved $1250 in fuel by doing this mod right when he got the car. Is that worth it?
Who measures modifications only by how much money they save anyways? Do you calculate how much new tires, or springs, or LED lights, or a new radio or speakers save you in fuel? I do it because its fun and its my car and I like to make it mine by doing fun things.
If its not worth it to you, don't do it.