Derek,
I won't speak for Ed.
What I am saying boils down to a few things:
1) To teach, one should know the subject well, and be able to effectively convey knowledge. Not everybody can do that. Some people are great at execution, but not at instruction. I've taught quite a few people to shoot, and guess I'm ok at it, as I've had several of those people ask me to teach their friends and family members. I could not begin to teach, say, horsemanship. I understand how to instruct, in general, but could not possibly demonstrate the techniques of that skill.
My lady, on the other hand, was a professional trainer of both riders and horses. She understands instruction, but could not teach people how to fly a plane, for instance.
I have buddies who are airline pilots at major airlines, who couldn't teach people how to tie their shoes. They know how to fly; some of them know how to play the stock market; but they don't know how to instruct.
2) To effectively convey knowledge, one has to be able to recognize the student's ability and knowledge level. Too simple an approach can lose a more advanced student, and too technical an approach can lose the newbie.
3) To get the student's attention, the instructor needs to have credibility. It may be hard to be viewed as an expert, by the person who has watched you order spicy Thai food even though you and she both know you'll be in agony later that night. Sometimes, lack of familiarity can be a plus. The third-party instructor is bound to have some skeletons, but we don't get a view into his closet, and what we don't know doesn't (normally) affect our perception.
4) I have taken my current lady shooting, and our dynamic is normally such that I feel comfortable teaching her how to do most things. BUT... I have been with other people whom I'd have been very reluctant to try to teach anything... I had my ex in a flight simulator one time, in a model in which I had over 2,000 hours (more than the sim instructor had)... she told me to just be quiet and let him tell her how to operate the controls; he told her to do the same things I'd been telling her, in much the same way, but she was willing to listen to him.
Even with the current lady, when a buddy and I took her on her first white-water trip, she was so nervous that we opted to hire a professional guide to teach her the ropes. My buddy and I were both experienced, but my lady was not raised around water, and her apprehension levels were too high for us to overcome. Luckily, the instructor made her comfortable, and she ended up having a blast.