Some people (not all tab readers) dont think that reading music is really that inportant when playing a guitar, thats may be it. To me, saying that is a bit silly. If reading music is important on a sax or a piano, then why not on guitar?
Being a musician and not being able to wright music is like being a poet without being ably to read and write whatever language it is you speak. Maybe you can come up with some cool profound stuff, but try getting it written down for another person, or getting it out into the puplic. It just doesn't work very well. Being able to read music is more important than a lot of people think.