Yes, it would be nice to include pitch and roll information in the EXIF data. And then get Adobe to update Lightroom etc use it as an approximate guide, if I wish. The D800 knows this because it shows us in the viewfinder or live view. Its just not writing this information to the file in a form we can read later.
OP asked about using the pitch and roll information to help with post processing. For those times when it may be confusing to work which is level. Or spur of the moment grab shots where the horizon is the last thing on my mind.
"We don’t see things as they are, we see things as we are" - Anaïs Nin