Just a moment, are we talking about open-boating here?
Look around you, you can see this everywhere around you. In every days life, you see certified guys, being a total disaster at their certification, and you have very skilled people, not having that paper.
Open boating is no exception on that.
Point is that, these days, you do need certification -insurance wise and people just like to see some paperwork, people believe it is some kind of guarantee.
No, a good boater isn't necessarily a good instructor. Nor do you have to be a very skilled whitewater paddler to teach the basics.
And yes, I do believe the best chance on a good instructor is that instructor who doesn't, or only briefly mentions his certification, but has tons of mouth to mouth comments.
A good instructor is, as has been said, someone who has thought about what he's willing to teach. How he is teaching will depend from person to person.
I neither like the proposition to take paddlers out to teach the basics. The basics are the fundamentals of our sport. Have novice paddlers learn the basics in a structured way. Take paddlers with you, to train in their basics, to build up experience before they can take their next instruction step. Training, building up experience is something different then teaching! I can take a flatwater course, but I'm not immediatly ready to go for a river course. I do need to train the learned skills. Take that kind of paddlers with you.
And yes, even at a certains kill level, it is good to step back down to the basics and refine them. It will be amasingly rewarding.
Teaching motoric skills is a hard job. It takes experience, and some good consideration. A good instructor is in my opinion also someone who has taken himself lots of instruction. Not to learn how to paddle, but to see how others teach, and learn some new skills in teaching that they can use in their own style.
And now I can start the same kind of story about plenty of other certifications in every days life, but I'll spare you that one