I have found that as I increase the platen gap I do see a decrease in acuity, and it is definitely noticeable.
I agree on some papers, especially the higher end gloss papers, but then I rarely need to use a wider platen gap on those. Generally I've found I need to widen the gap on things like canvas and some textured art papers, and in those cases I really can't see much of a difference.
Certainly there is no need to widen the platen unless there are issues.