I have always felt that double-tracking leads is kind of unnecessary for a lot of reasons...One being the difficulty of nailing it twice, this can be near impossible to play it exactly the same two times depending on the style of the leads (shred, etc.). The noise issue, I mean, sometimes one take will be perfect, and the next best one (say he nailed it again, and he just can't play it anymore), has noise here and there, and not in a good way...what then? I'm referring to situations where editing out things isn't an option.
Unless the guitarist can nail it twice exactly the same, I usually find a way to convince them to leave it at 1 track. That said, I have double-tracked my leads before, several times, and I have had a lot of guitarists come through who I made them double-track theirs just because they were really good and could pull it off. It really comes down to how clean the guitarist is. Thats my view anyway.
~006