Hippies come in two flavors in my experience.
Really cool mellow folks who just want to live and let live, eat some biodynamic local organic pizza, and smoke a bowl. They wouldn't do it because they don't want to harsh your day, and are generally cool folks.
Super angry trust fund kids who protest everything, can't calm down, and don't get the happy dynamic that the other group does. They wouldn't do it unless there was media coverage and it was a protest. They might steal your stuff, if you represent something they don't like. Generally they just want to be seen, and make others (mom and dad) angry to make their point. They verge on anarchists.
I have never heard about any instance of hippies getting down in somebody else's tent. I have run into lots of the first group out here on the trails when I was going to school in Eugene. I don't always sync with their dogma, but they are usually good neighbors on the trail. And genuinely concerned with the world around them, for as long as they can focus their attention span.
Now if you park an unoccupied tent at the Oregon Country Faire for a day, I think you have a higher probability chance of seeing this happen. Just hope that they practice LNT. :)