Well - within what we'll call the world of the 'fine art photograph' (see my comment about the 'old guard' above), such a seam - and respect for truthfulness in materials, isn't really part of the language. In fact, it's downright alien. However, such gestures and playfulness with the media at hand ARE very much part of a language, and very much at home, in that exterior world that we refer to as 'visual art', and commonly find such moves in modern and contemporary (these are different epochs, in my book) painting and drawing, mixed media, etc... Wall's work is very much part of that world. And he takes his referents from that world. He is very much aware of the deeper connotations of such gestures - as, I think, were his intentions. It would be naive to suggest otherwise. Look at what he has to say about it, here, on this page, under "picture for women":
The seam running down the middle of the photograph is apparent in some of Wall's large-scale pictures, where two pieces of transparency are joined. The fact that it serves as a reminder of the artifice of picture making is something that Wall has come to appreciate: 'The join between the two pictures brings your eye up to the surface again and creates a dialectic that I always enjoyed and learned from painting... a dialectic between depth and flatness. Sometimes I hide it, sometimes I don't', he has said.
Okay - well, I suppose that pretty much says it all. He enjoys the tension between pictorial space and the space of the artwork.
nothing wrong with using language that is outside "the system."
not every photograph has to speak the "formal language of photography."
the "system" is broken if every photograph has to be compared to an adams or weston.