Just about a month ago, The Spectacle ran a post called When Writers Don't Read. In it Parker Peevyhouse wrote about advice she's seen to writers that they should not read fiction if they write fiction. If you go back to the person who originally gave the advice, her reasoning was 1. Reading fiction takes from writing time; 2. Reading nonfiction of various sorts expands a writer's knowledge base (what she knows and can write about); 3. Reading fiction could mean that an author's work becomes derivative of another author's. Peevyhouse and her commenters didn't agree with this process.
While I think the original advice has legitimacy, I would argue that 1. Reading anything will take time from something else. If your writing time is very tight-- say, you have a full-time job--the time you spend reading nonfiction to expand your knowledge base will also take from your writing time. Omitting fiction isn't going to help you much here.; 2. Reading fiction adds to your knowledge base in terms of studying how other writers deal with the elements of fiction--how they plot, create characters and setting, how theme is handled, etc. For writers, reading in their genre is a form of study.; 3. No writer wants to be so derivative that it shows. Nor do most of us want to plagiarize, intentionally or not. But there are probably a finite number of styles of writing and most of us are going to prefer to write in a limited number of them--styles that we learned from our reading at some point in our lives and adapted to ourselves. So to some extent this is going to happen unless we've never read anything in our entire lives.
One of Peevyhouse's commenters mentioned children's writers who claim they don't read children's books. I think this is an example of problems developing from not reading in your genre, particularly when you're talking about writers who are new to the children's lit field. This is how you end up with people claiming they're writing for children because there are no kids' books like the ones they write. Given how many thousands of children's books are published each year each year, that's extremely unlikely. Ignorance of the field you work in doesn't inspire confidence in just about anybody.
The same must be true for writers in other genres. If you think you're the first person to write a novel about a woman coming to terms with aging and her children growing up and leaving home--and say so--you definitely should have spent some of your fiction writing time reading fiction.