why don't the image files from Nikon, Canon, etc. have information in the image files specifying the exact orientation of the camera (at least roll) at the time the picture was taken?
Depending on the subject, background and perspective issues, it is sometimes a bit difficult to determine the horizon reliably. Sometimes trees, columns and beams in the middle of the photograph are not really horizontal or vertical. It would be nice to have this information ready for the post-processing.
The technology for this should exist already; game controllers and smart phones know their attitude, e.g., to drive a car simulator. However, currently only coarse portrait/landscape orientation is supported.