Sitting here in 2018, I can't emphasize enough on why Mixing is just as much a composition tool as putting melodic notes together. Most people would say mixing is cleaning audio and putting some EQ, Delay and reverb however, the choices that you make while creatively mixing can change the entire aesthetic of your composition.
While composing for picture, panning and sound effects that navigate the music become so much more important as they resonate with the action going on in screen. If you are a modern day composer then knowing how to manipulate audio is probably more important to you than being able to score out notes and creating sheet music. Even back in the day when the first scores for Disney cartoon's were created classical instruments such as the Timpani were used to mimic the visual action of the characters. Contemporary composers more often than ever work solely by themselves, we all do not have the luxury to just write the music and rely on someone else to mix it therefore it is more important than ever to add that skill set to your tool kit. A Logic project can quite literally replace written sheet music.
We have been conditioned to listening to highly produced music, I truly believe that the way an instrument sounds is a part and parcel of writing a piece of music. Crescendo's, diminuendo's and dynamic signs such as forte, piano, mf and so on have been pretty much replaced by automation, fades and audio manipulation.
I find myself making so many choices in the mix that define the feeling and emotion that my music would eventually generate. There are very blurred lines between production, mixing and actual composing in the world of electro-acoustic music and I believe in order to find your "sound" it is incredibly essential to access every avenue possible. At the end of the day, mixing can be just as much a compositional tool as an instrument.