The necessity for reading music depends entirely on what one wants/needs to be able to do.
Reading music at a high level generally takes years of study, particularly for piano.
Depends also on the instrument one plays; there're tabs for guitar, treble and bass clefs for piano, treble or bass clef for wind, brass and string instruments, lead sheets and chord charts for guitar or piano, full score for orchestras..... Different kinds of music reading exist.
For most genres, a lead sheet or tabs are enough. And some musicians just use their ears, and sometimes that can be enough too. Except when one has to play a song one has never heard before.
Yeah, it really depends on a lot of things....
Well it really depends on what you're trying to do. Are you just going for some chords that are supposed to be "the right key", or is your accompaniment worthy in it's own right?
I'm trying to imagine even a concert where the pianist suddenly plays it's own thing that sounds just a hair different then the recorded album version- and suddenly fans would say, wait a minute, that's not "right", because then all of a sudden people will insist on the exact reproduction they heard before.
Of course in classical music you'd get booed of stage if a pianist would just make up some chords to go along to some really known Schubert or Mozart song, because then the piano is so much more than just supplying a chord so that deaf singers know what key they are in. (just kidding...)
Depending on the music your accompaniment will live in it's own right.
So I'll say it depends on what you're trying to do.
It's also quite the advanced skill after years of study to be able to reproduce a piece of music strictly from paper without ever having heard it- being able to read what the composer actually meant which in turns means that the person writing it down needs excellent skills as well.
Imagine music from 200 years ago- there was no recording, those folks are long dead- but we have notations of their music and hopefully can try to reproduce it.
I wouldn't discount the ability to read and particularly write sheet music- imagine an almost deaf composer who simply won't be able to reproduce his composition by ears anymore...
That's a very rare skill these days and if I see what a program does to my own music. (playing while GarageBand spits out a score that really doesn't have much to do with what I actually played...)
I'll say it's a skill that will be needed in the end if we want to be able to reproduce music in it's entirety.
I am constantly amazed at some of the arrangements that MJ was able to come up with without being able to read or write sheet music. That's just amazing to me, I will forever take my hat before him for hearing an entire arrangement like that in his head.
He had people putting it down for him so in the end SOMEONE had to do the deed there too.
He also worked on some music that was going more into the classical side of things- and was looking for someone to orchestrate, another lost art form that's not really thought in mass quantities.
Also being able to read sheet music is still a whole level away from being able to correctly write it down....
I wouldn't discount that ability either. You can compose wonderful music and then might have a heck of a time letting others reproduce it when you're the only being able to play it...