If they put 3 or 4 Pro Tour's in the same 2 month period using the same format for all of them, I bet you would see more people trying to get their decklists withheld.
Withholding a decklist at a large event is not really an option. At a premier event there is a lot of scouting that occurs.
For instance, at nationals, there were several teams which had everyone's deck and variations after the first three rounds. Scouting is legal, and a very common practice. In fact, Wizards releases the top eight decks at these events before they are played out.