Sheet music is for some musicians an unnecessary abstraction: For these musicians, music should be heard and not seen.

For them, music should enter the brain via the ears (as opposed to via the eyes), and music should be expressed rather than written or printed.  These musicians often site natural sounds and noises as sources of melodic inspiration.  Does this imply that sheet-music is an abstraction written for, lets say, relatively non-musical bystanders?

Not necessarily.  Musicians who can also obtain their melodies from sheet-music have an extra source which can expand their playing capabilities, a source or medium which exists outside of his or her own sphere of audio inspiration.  For example, a musician with the ability to quickly learn to play a complicated Beethoven piece from written music can become a better musical performer from a complex source existing entirely outside of his sense of sound and intuition.

Singing Bassists who can visually ascertain from sheet music the locations where bass-notes and vocal-phrasings intersect are able to learn music-pieces faster, and of course more accurately, than singing-bassists who must mechanical (or use software to) slow down recordings to find these locations in order to train them.

Of course, in band-settings or in any collaborative music experience, the Singing Bassist who exclusively “plays by ear” will be faster to learn a new song, faster to react to changes, because that Singing Bassist habitually relies on the Ear to start any musical endeavour, from tuning the Bass up to writing a song.

Musicians able to read music as well as “play music by ear” take advantage of a broader tool-set than musicians who only read music or who only stubbornly “play by ear”, and are thus able to musically progress faster than their uni-sourced counterparts.