david weaver
I guess it's not a question for me of whether or not they're complementary, or whether or not they can be used with good effect (sure they can, the mouth and the cap iron just can't close off completely) but whether or not they are redundant when the cap iron is used properly. i think they are. when the cap iron is used as a back stop to prevent tearout in a deeper than expected cut, then the mouth may not do much in any cut.

But we're not talking about plane mouths 1/4th of an inch, rather mouths that are allowed to slip back to a 16th or a 32nd.