Good question.
I suppose it has much to do with how we who have done alot of "compatible" mixing monitor.
We used to do it more, but these days I think it's pretty much down to mixing for mono TV, and what I do is to switch back to a mono monitor alot to check my stereo mixes. Generally speaking, what predictably happens is that the 'stuff' on the sides is mixed louder when you do this. This is not hard to accomodate by panning some things that should be less dramatic sounding in.
George Massenburg
