First of all I was using it before named volumes were introduces in Docker I think (thus I was able to able to upgrade my own installations).
Also in my opinion it is easier to manage in this way - less words, previous versions had much more volumes and it would have being very verbose to declare them all separately as well as would complicate upgrade (and backup/restore data too).
Having that said, there is nothing that prevents you from using named volumes, it will work perfectly fine if you prefer to do so.
First of all I was using it before named volumes were introduces in Docker I think (thus I was able to able to upgrade my own installations). Also in my opinion it is easier to manage in this way - less words, previous versions had much more volumes and it would have being very verbose to declare them all separately as well as would complicate upgrade (and backup/restore data too).
Having that said, there is nothing that prevents you from using named volumes, it will work perfectly fine if you prefer to do so.