I know, I've talked about this before, and so have lots of people. But it really is a bit like the elementary teachers explaining over and over the reason kids need to read or be read to 20 minutes a night.
Because if we don't, nothing else we do will matter.
Sure, you could be a prolific writer of brilliance, but there are nuances within each genre that a writer MUST be aware of. For instance, the difference between romance in MG and YA, or closer to my writing home, the different emphasis between romance and women's lit. We have to know the kinds of plot structures that work in different genres and we have to know the appropriate character configuration. Older readers can appreciate multi-character plot lines, whereas younger readers would get confused with too many characters.
I would argue the biggest reason to read the genre we write is to use it as a guide in our own writing. This is our chance to see if we are on track, if we do recognize the quality stories, appropriately deep characters and the like. But when we are reading in our genre, we are allowing our brain to be filled with the kinds of things we are trying to create. I think most of my readers have been out of high school for a while and I'm equally certain each of us have some sort of knowledge base that may have been reasonably sturdy for some time, but now is crumbling and weak.
For us to be able to pull from the creative tank, it has to be full. Doesn't it make sense to fill it with what we need? Sure, we can read other things at times, but I really truly believe we will export according to what we import. I don't think this means someone is locked in forever - I think reading preferences should change as writing preferences do.
And if you don't read what you are writing, why are you writing it?
Have you noticed a difference in your writing based on your reading?