Nice article. Some thoughts on it:
Here is an interview with Ansel Adams http://www.youtube.com/watch?v=ZND3eczqoIA
and he expresses very well, I think, how he deals with what Mark Schacter discuss in this article.
Ansel also express a fascination with the possibilities inherent in the digital capturing and editing of pictures.
This is not cheating, this is creating art. Not so much rules although I guess every photographer has his own rules.
Andreas Gursky has other rules than what Ansel seemed to have, but the end result in mind is the same, to create interesting art.
In some cases Andreas combines several exposures into a single image where Ansel, as far as I know, never did this.
From what I have seen Andreas combines different exposures and selection from the same scenes but a few exceptions like his photo of the miners clothes where he even have a separate shoot of some workers to put behind the chains that are used to hoist the clothes up while the men are working in the mines.
Is this cheating? Yes, certainly if your rule is that what's in the photograph all existed together at one precise moment in time. Does it matter? Not really as it could have.
Where is the limit to this? Collages of totally unrelated pieces in time and space to create pictures? Well for me I don't go any further than Adams did, but I have enjoyed an exhibition of Andreas pictures in large print. I think only you A photographer set the limit and it does not really matter what the rules are for other photographers or viewers.
One example of a photographer who seems to be very popular on 500px http://500px.com/Alshain
composes his images quite freely from different sources.
Is this cheating? Not exactly my cup of tea, but as an art form it isn't.