I completely agree that experience and consistent diving are much more important factors than certification level alone.
That said, I have two things I'd like to add regarding training:
1 - Different people learn in different ways -- while many of us are comfortable with self-learning and applying experience as lessons, many do better in an mentor-apprentice or similarly structured environment. The instructor led courses many agencies offer should be designed to mimic that mentor/apprentice experience. Unfortunately, as many here pointed out, many instructors out there do not make very good mentors 
2 - As UW photographers, many of us tend to push the limits of recreational (or technical as the case may be) diving. If you don't believe that's true, go check out the recent thread regarding dive computers
. As such, it is important - IMHO - that we understand the physiological, environmental and physics aspects of diving and how these affect what we do. As an OW diver I was conscientious enough to realized that what was taught in the course regarding the science of diving was not enough. I took it upon myself to further my education with books and by interacting with knowledgeable divers. Even then, there were things that I didn't know - or had no opportunity to experience in everyday diving - until I went through the DM course.
I am very fortunate to live in the an area where the diving occurs in cold water, low visibility and strong currents. All of my training and most of my experience takes place in this environment. It makes the trips to the nicer destinations all the more enjoyable.
So what level should a diver be before undertaking UW photo? Who knows! Like many things, it is up to the individual and his/her natural skill/experience and drive to improve. But I don't think that training should be discounted as a valid avenue for some to get there. The important is to realize that it is not about collecting cards, but expanding your knowledge.
For the record, I'm not a diving agency rep - sure sound like one on my rant 