A contributor has replaced part of my family tree with different persons who are unconnected to my family. It seems pointless in spending huge amounts of time building your tree for an unknown contributor to eliminate all your work and it seems you can do nothing about it.Crazy
Yes, it is frustrating when changes are made that you don't agree with. Please remember however that FamilySearch is the only public tree available and so others can add to the tree and that is helpful since they are probably related to others in the family. But as you indicated it is frustrating sometimes as well. Here are some links to articles that may help you: (note that there are related articles at the bottom that can help as well.)
Since FamilySearch is a public tree you may also want to look at other Family Tree Management partners with FamilySearch where you can build your own private family tree and have that on you own computer. The great thing about these partners is that you can share information back and forth with these Family Tree Management partners with FamilySearch electronically. For example if there is information that you find someone has added to FamilySearch that you agree with and want to bring into your private tree you can do that easily without retyping the information. And if you have information in your private tree that needs to be added to FamilySearch (more family members, sources etc.) you can share that with FamilySearch for the benefit of other of your cousins and their decedent's.
Here some links about the Family Tree Management partners:
I have found that collaborating with fellow cousins that are working in the FamilySearch tree is very helpful. You can message them through FamilySearch and find out how they are related to a certain ancestor and discover more sources, stories and relatives. Usually people want to make sure that everything is correct and sometimes names, dates and places can be similar. So working with related cousins is a good thing.
Hopefully this information can help you in understanding the benefits of a public tree.
Thank you for contacting FamilySearch. It has been a pleasure working with you. Best wishes in finding more information about your family.