I spent Friday, Saturday and Sunday nights at the stones. I thought the travellers were a little boisterous when they arrived on Saturday night but I didn't witness any defacing of the monument over the whole weekend.
Regarding fires. There was one set outside of the main site on side the southern rocky outcrop. The fire within the circle was contained within a couldron-like thing which was carried around the site and left no obvious marks on the ground.
As for Callanish becoming a place of pilgrimage for travellers, I was discussing this with a few folk, apparently during the years when Stonehenge was closed at solstice there were an awful lot of travellers at the stones but these numbers have decreased over the past few years.
The wee-frees were very chilled, they came, they sang, and then they left. I found their singing added to the ambience.
I gotta say that I've witnessed a few circus-like gatherings at stone circles and I reckon that apart from a few shouty travellers the whole thing was very mellow.