According to rule 801.04 Courtesy:
"B. Players should watch the other members of their group throw in order to aid in locating errant throws and to ensure compliance with the rules."
and
"G. A player violating a courtesy rule may be warned by any affected player, even if from another group, or by an official. The player shall be assessed one penalty throw for each subsequent courtesy violation of any type in the same round. Repeated violations of courtesy rules may result in disqualification in accordance with Section 3.3 of the Competition Manual."
So if I drive but then fall down so I don't see where my disc lands, get up and ask the other members of my card if they saw it and someone says "I wasn't looking," I could call a courtesy violation warning on that person. I have played some pretty vegetation-thick courses, so I can see where it could be helpful if everyone is watching a drive. Also, if a player goes OB far down the fairway on their drive, or there is a question of when/whether the disc was IB, it is also important that other players were watching.
So my real question is, have you ever seen this called, or would you ever consider calling it? I have definitely been guilty of not watching other players throw before, but it just occurred to me from a post in another rules thread that it is actually a potential courtesy violation.