I too had a public tree on Ancestry which worked well and attracted the many cousins that I now treasure as email contacts all over the world.
One day I found that matches were being thrown up for a private tree, for about 20 of my relatives across lines. I was quite excited, expecting to knock down some brick walls, so I contacted the owner and got no answer on any query. On further investigation I found all my photos and the stories I'd written on various relatives were copied as well. The thing is, as much as you know that people can copy the contents of a public tree, you can't be prepared for the kick in the gut you feel when that happens and despite contacting thems everal times, there is no reply or acknowledment. My tree is now private and like others corrections made which won't be shared. This person also has 12,000 names in his tree.
Last week however I found my photos on another tree, this time with 14,000 or so in it. I emailed, expecting no reply and actually got one. This person explained that my relatives were not in his direct line, but came up in an indirect way. He said he likes to collect everything he finds a match for to help out any others that might be searching. He asked me if I would mind checking that what he had was accurate, which I did and he made the corrections I suggested. Not my preferred way of doing my tree, but I was immediately more at ease because he had the courtesy to reply and explain.
At the end of the day, any way that makes sure our relatives are remembered and their details accessible for those searching for their ancestors is a good thing, no matter what!
Jill