Only thing that should be private are things that have only one or no specific use.
For example, those bricks in the Titanic. They were only made for specific parts of the titanic and there aren't really many other uses.
Elm's trees, though, are very useful. They can fit in with many builds, look nice, and are trees. People could really use them and make awesome things, but because elm wants them private for some dumb reason, not many people know about it and will continue making builds that look pretty bad because of the default lego tree. People usually aren't trying to build lego-like, they are trying to build realistically. They want realistic trees to fit in, as well as other bricks that may be private that I might not know about.