Broadly agree with you; I've no problem at all with people wanting to spend all day at a site if that's the way it works for them. There are some places that demand you spend more than a few minutes where I've been blessedly lucky to have Callanish, Brodgar and Machrie Moor all to myself for as long as I wanted, on each occasion over an hour. Whether you're there for five mins to take snaps or like to take longer (including taking a few photos) you're entitled to the same consideration you would give others and that includes being aware enough not to encroach if you don't have to. That's particularly the case at the smaller sites where I agree that generally speaking the bigger, the more having other people in shot is par for the course.
Strikes me you're the aware sort so I shan't mind if I encounter you on one of my visits......