My worst ever example of this was at Boscawen Un too, funnily enough. One year we went there the day after the Summer solstice (which had been very rainy) and we got there to find a group of 7 or 8 people in the circle, with finger cymbals and chanting. We'd walked a few miles over the fields to get there so it was a bit disappointing but not unexpected during the summer.

So we waited in the (then) long grass away to the side to let them do their thing. About half an hour went by, during which a few more visitors came and waited patiently for the "ceremony" to finish.

It went on a bit longer, then finally came to an end - we'd been there getting on for an hour and still had quite a long walk to get to where we going to be catching the bus from. Anyway, they could obviously see that we and the other visitors had been waiting quietly for them to finish. Rather than vacating the circle, they then proceeded to stay inside the circle and started chatting about jobs, kids, etc. I thought this was massively rude when everyone had waited for them for so long.

Anyway. They still didn't leave, so we did. As we left the circle (not the way we went in) we passed their minibus parked in the layby on the main road. Bloody weekend pagans :)

This was one of the reasons why I rarely like visiting sites when there are other people there. It's a reason I hardly ever go to Merry Maidens and much prefer remoter sites where you can't just park up next door (I appreciate not everyone can easily access more remote places).

Another reason I don't much like visiting "show sites" is this (see comments as well):

http://www.themodernantiquarian.com/post/109378/fieldnotes/sunkenkirk.html