[derekp7]

This is something I never understood. If I put a piece of code out there under a BSD style license, how can others lock me out of my code? Yes, they can add non-free code to their copy and distribute it, but my original copy is still there. Unless they add a feature that I was also planning on adding, and they patent that feature, which would keep me from adding the same functionality to my copy of the code. But how likely is that to happen?

[yarrel]

You can find yourself using a computer that is running code you've written but that you are not free to modify because you are either only provided with binaries or actively prevented by the software from modifying it.

See iOS for how common this is, which is to say very.

[ArneBab]

They can make an unfree project which pulls users and contributors from the free project, essentially killing the momentum of the free project.