That is simply not practical for anything except toy projects due to the complexity it adds, and is rarely necessary anyway. The most common case is a GPL proje

That is simply not practical for anything except toy projects due to the complexity it adds, and is rarely necessary anyway. The most common case is a GPL project importing BSD licensed code with minimal adaptions, e.g. just enough to integrate the code. The authors of BSD-licensed code are clearly comfortable with people adapting the code for proprietary purposes without submitting their changes, so there shouldn't really be a problem with copyleft projects doing the same. And if the authors feel otherwise, they should use a copyleft license themselves (perhaps the LGPL in a situation like this).