I was shocked when 4 announced its built-in library designed to take easy advantage of notation, and it can have really impressive results. However, depending on your workflow and needs, using commercial libraries still has its advantages. I _hate_ using notation programs, they've always been so cumbersome to me. I either write music with pen and paper, or when I need to put stuff down electronically, like for a score for example, I go straight into a DAW and piano roll. I didn't investigate it thoroughly, but I don't believe MuseScore is compatible with this. I've also read complaints that sometimes the MuseScore libraries have a "good side", and can have a hard time "following directions" when you're using it in certain scenarios.
That said, they've done an overall outstanding job, and I'm happy that people all over the world, if they have the tech specs required to run MS4 libraries, now have free access to responsive and good-quality libraries. It's truly remarkable.