The problem is that many Open Source projects aren't created by an individual, or even a single company. Large projects have hundreds of contributors. If a project requires copyright assignment, all these contributors have fewer rights than the maintainer of the project.
If a project doesn't require coypright assignment, and uses a single license, all contributors have equal rights to the collective work.
It's some sort of empirical law that almost all of the code in free projects is written by at most a handful of all contributors. If this law holds, and I can think of only few examples where it doesn't, it seem fair for the tiny minority to benefit the most from selling exceptions.
What the FSF does when requesting copyright is a two-way agreement. The FSF drafts a contract that promises that they will never use the copyright aassignment for creating non-free software. Similar contracts could be drafted that agree to share revenue proportionally and provides clauses for arbitration if this proportion is in dispute.