I agree. BUT for me the most important skill in mastering is deep listening and detail rating of the music. Therefor it can not perfect been done by the writer or mixer as you focus is always polluted during the proccess in writing or mixing before. How can you say its 0,5dB too much bass when you mixed it like this for weeks? I mean, you need a mastering engineer that understands your needs and words and music, with fresh ears and you two needs to merge to a whole new level of competence! ...all the tools are just homework ...ok expensive homework but without the things I wrote before ...your mastering will not succeed!
Regards, mate
Bodo