I can understand how you might feel poached if someone wanted a bunch of your information in order to publish it. But, I imagine that most people requesting a look at another's work are just trying to make sure they are or are not on the right track.
I do not consider historical records proprietary information. Often I'm trying to make sure I have the correct John Doe of the four living in the same town. My ancestors, all named their children after each other, had huge families and lived within a mile of each other for years. It's baffling to me why someone I think is a probable (albeit distant) cousin of mine won't let me even look at their tree. How does it hurt someone to let me gather information? After all, I am trying to be as accurate as possible too.
I don't have the ability to travel all over the world to gather information. I wish I did. I certainly don't include information in my tree that is questionable. I firmly believe there are unnecessary brick walls in my research because someone else feels my reasons for wanting information are not legitimate enough or that I'm too distantly related.
What do you gain by not sharing? Thanks, this is a huge pet peeve of mine.