Drop-frame 525/29.97 timecode gives you an average of 29.97 frames per second. This is imperfect (29.97 is not equal to 30000/1001), but it is a lot closer than 30. Drop-frame 525/29.97 timecode will give you an error of two frames per day, compared to 24 for non-drop-frame timecode. Studios take it as part of their regular operating procedure to reset their timecode generators occasionally, and to never trust timecode intervals longer than about 12 hours.