Some clubs have operating time slots for the members, but in the spirit of celebrating the hobby as mentioned above, we just try to "keep the seats warm" as the ebb and flow of the event goes on. Some folks are avid contesters, while others are happy to operate a couple hours at a time as needed.
It works out pretty well and it keeps it feeling like a fun weekend for everyone.
Yes, it does--until you get the few that keep on with thought that field day IS a contest. It makes no difference if the club or group that is sponsoring that event says otherwise, there are still some with "oh, get that one, I think it's a new one..." or "Don't bother with that one, we've already got them" and in doing so, short circuit the fun of the event--and the attitudes of those who aren't yet comfortable getting on the radio or are first time operators who are put off because they see something that they're not comfortable with.
One year when I was responsible for running field day for a club, I put my foot down. I said OK, lets not get contest minded here--we're here to have fun and to help people get their feet wet, not to score the highest we possibly can. You should have seen and heard the looks and comments from the few who didn't agree.....