A strong copyleft license benefits the copyright holder the most because they
are dictorial in forcing you to contribute modified code back, but can also
force you into a commercial license that you may be forced to pay for and be
beholden to the copyright holder. This is in contrast to a permissive license
that can be used without restriction. A strong copyleft license is "free" in
that offers freedoms to the people RUNNING your software, but not the
developers who actually DEVELOP software because they have limited options.
Since I used MongoDB in a previous example, it uses a strong copyleft license
but if you don't want your custom changes forced to be in the wild, 10gen,
the copyright holder, offer a commercial license to take away the copyleft
nature: http://www.mongodb.org/about/licensing/
Some companies take it an extra step like Canonical with Mir's GPLv3
licensing. It looks great on paper, but it is only GPL if you sign their
custom CLA which allows Canonical to take your contributed GPL code in Mir
and change it to another license if they so choose.
