I'm firmly with those who think that photography, like any art form, requires the twin disciplines of craftsmanship and conception.
Language requires rules - spelling and grammar - to aid communication, and writers use these rules to guide their readers. A photograph is simply another means of communication, and, like language, has rules that photographers use to underline what they are trying to say. In our case, the grammar of photography is composition, whether captured directly or created in the darkroom by, say, dodging/burning.
That said, rules aren't laws, so underexposure or slanted horizons are fine so long as they are deliberate, or at least don't work against what the photographer is trying to say.
However, knowing how to create a technically proficient and well composed photograph isn't enough: the photographer must have something to say, and all the elements in that photograph should support that idea.
If one of these two components is missing, the photograph will fail, in my opinion:
• How many poorly composed "street" shots do we see where we're not sure what the subject is and our eye is distracted by a several objects or led out of the frame?
• What about those myriad boring landscapes with the horizon a third the way down and a lone building or tree plonked on the intersection of a third?
Lacking craft, the former fails; lacking vision, the latter fails.
Ash states that he uses the camera to document the here and now, and that content in this context has primacy over technical proficiency. I can agree with this inasmuch as it's better to have any photo rather than no photo if the aim is to document Britain in 2006.
However, I'm sure Ash would agree that, none the less, the ideal documentary shot should communicate what the photographer is seeing and
also be well-composed and technically perfect. He links to a photo of his:
http://www.rangefinderforum.com/photopost/showphoto.php?photo=47386&cat=500&ppuser=5280
This communicates what Ash has seen, and is also well executed; for example, the flat lighting matches the sombre mood (it wouldn't work in colour on a sunny day), and the elements in the photo support each other (e.g. the lighter areas create a circle for the eye to travel around, and the two rectangles in the top left and lower right create dynamism, as does the slanted horizon). (I didn't realise we had such shops here <shudders>.)