Te debate about reading sheet music rages on I see!
I'll make the language analogy. Reading, writing, speaking (playing), and construction (theory).
These four areas make up the use of a language, and translate itself into fluency. Without a working knowledge of all areas, a person
can be fluent, but for deeper understanding, you need a working knowledge of all areas. That doesn't mean you have to read like an orchestral musician, be able to write like George Gershwin or improvise like Satch, but a knowledge and ability to do these areas to some level is needed.
For instance, I've been improvising since I was 13, over 12 bars with my brother. That's the speaking part, and it came first to me. Reading, writing and construction came together, where I learned how to write riffs, songs, and delved deeper, discovered what worked, what I like and what I didn't, and did all of this through tab, but with an accurate knowledge of crotchets and semiquavers. Treble clef notation came after when I went down the classical route, and I gained a further insight into melody, harmony and construction by being able to see it in piano pieces, violin pieces, etc. On the writing side, the best advice I got was to write without your instrument. That's a true test of how much of a musician you are to me, being able to put ideas down on paper, not necessarily in clef form, but to be able to hear it in your head and relate that to writing.
It's true that a whole host of contemporary musicians don't read, and possibly can't read sheet music, and to discredit their input based on this is silly, but there's only one Jimmy Page, Jimi Hendrix, etc. Collectively, these people who are ground breakers make up less than 1% of guitar players worldwide. If orchestral musicians were to ignore the ability to read because Yehudi Menuhin, or Nigel Kennedy didn't read, then the orchestra as we know it wouldn't exist, and a lot of that music would go unheard. People say it's different circumstances, but I can't help thinking that if guitar reading was more prevalent then there wouldn't be so many covers bands around with bad approximations of songs.