Readers need only a Reader Hat. Editors need a Reader Hat and an Editor Hat. Writers need a Reader Hat, an Editor Hat, and a Writer Hat...and some knowhow as well: what each hat is best for, and when to change them.
Writers need a Reader Hat because it's by reading (and listening to stories) that we learn how to write stories. Good readers have a head start on becoming good writers...readers who enjoy a lot of different kinds of fiction know from experience what makes a story interesting/exciting/attention-grabbing to readers. With a Reader Hat on, writers can experience their work as readers do, and see if it provides the experience they wanted to give.
They need an Editor Hat because no writing is perfect straight out of a writer's head. Some of the things an Editor Hat points out are not directly related to reader satisfaction, but most are related to at least some readers' satisfaction. The Editor Hat doesn't just find problems/errors/clumsiness in the work, though...it analyzes and problem-solves, helping writers fix whatever problems it finds in the most efficient and effective way.
And of course they need a Writer Hat for the actual writing--and the Writer Hat has to fit comfortably and stylishly (defined by that writer) so that the writer can wear it all day every day for months on end. The Writer Hat does more than cover the hours spend writing...it's what notices bits of "business" and dialogue and scenery and everything else a writer needs to put into a book. The Writer Hat is a vampire's hat, a scavenger's hat, an eavesdropper's hat, always out to steal bits of life and tuck them away for later use.
Most writers should wear the Writer Hat most of the time they're not reading someone else's work or editing their own. But when reading, they should hang up the Writer Hat and put on the Reader Hat. Why? Otherwise they'll be trying to re-write the other writer's work, or comparing it to their own (to their despair or their gloating satisfaction: neither is healthy.)
Occasionally, it's worthwhile to read with the Editor Hat on--to analyze, for instance, how a better writer accomplishes a technical feat that the writer is struggling with. That's like watching a great athlete or artist to figure out how to do that jump or make that curved stroke with the brush. But mostly, when reading, wear the Reader Hat.
The Editor Hat goes on only when analyzing a work. We're taught to use the Editor Hat (or something like it) in school, when reading and analyzing the texts we're handed, so the Editor Hat tries to leap off its peg and onto the writer's head at times when it shouldn't be worn...take it off and put it back. With the Editor Hat on, its negative approach to text (disassembling it, analyzing it, noting problems with it) overshadows its positive possibilities (solving those problems it found) unless the writer is very firm with it.
Only rarely can (or should) writers try to wear two hats at once, and the most dangerous combination is the Editor Hat with the Writer Hat. With experience, it's possible (sometimes, for short periods) to hold both goals in mind during a revision--switching sides of the head, as it were--to notice and fix problems in one pass. Works pretty easily for one-word or two-word fixes, and becomes progressively more difficult as the text to be analyzed and re-written gets longer. It's best, if the revision requires rewriting more than a paragraph, to mark the manuscript with the Editor Hat on, then take it off, hang it up, jam the Writer Hat down over both ears, and do the rewrite with only the Writer Hat. It may take a coffee break (or chocolate break, or short walk) to ensure that the Editor Hat doesn't contaminate the Writer Hat's ability to write the new stuff.
Just came from a weekend of struggling with this very problem.