I have heaps of side branches in my trees. I love following all the family lines. And by doing this, I have found many "distant" relations are still in contact with my direct lines - sometimes living at the same address! One of my family lines all lived in the same small area for many, many generations and the various branches still knew each other, lived together, had businesses together, supported each other in bad times, etc. You bet they are all going into my family tree because they are family even if not in my direct line.
In one case I was able to break through a brick wall by identifying a second cousin relationship and following the generations back. The second cousins came out to Australia together on the same ship! Those side branches are incredibly useful.
In another case, by understanding all the relationships between all the different in-laws, side branches, very distant relationships by marriage, I was able to break through another brick wall. They were all connected to each other, in each other's wills, going through chancery court, suing each other's estates, and most of them weren't even blood relations (i.e. in-laws of in-laws)! But the whole picture proved relationships and provided what I needed to break my brick wall.
It is absurd to think that people live in isolation. They are the product of ALL the people in their lives - friends, relatives, business partners etc. Family research that covers only the names, dates, places without investigating the stories of their lives is just genealogy. I'm interested in more than than. I want to see a little way into their world. And that means understanding the other people in their lives.
A very important person in my husband's life was not even a blood relation but a very, very close family friend. I intend to do some family research on his life. He had no children of his own but was a wonderful man, an important influence on my husband in his childhood, and he deserves to be remembered by his "adopted" family.
I'm finding cousins all the time researching the same family trees. Just because they have information about my family tree is not stealing. They are just doing the same thing as me. Researching what is already in the public domain. I've even found academic articles about ancestors by people who were doing a thesis and are in no way related to my family tree. And I don't see that as stealing. In fact, it was incredibly helpful. Collaborating with other people can sometimes be the best part of family history research.
I do not "own" my ancestors. They were their own people who lived their own lives. I'm just trying to get to know them a little.