I read an article saying that jackd (which routes audio between programs, and offers a cross-program audio transport/play/pause/timestamps) arose to bypass the issue of the GPL making it difficult to link multiple audio plugins with incompatible licenses into a single DAW. The article said that session management (saving and restoring the state of multiple independent programs, as well as routing between them) proved to be the biggest weakness compared to audio plugins. Unfortunately I can't find the link to the article anymore :(