You don't edit your own, do you? I mean, a couple of drafts, sure. But actual editing, the final process, should never be done by an author upon his or her own work. A patient doesn't perform surgery upon him or herself. It hurts too much to cut stuff out. Somebody else has to do it.
That depends on the writer. Or, perhaps, what kind of writing. Take some kind of popular genre fiction, say, oh, crime fiction, or straight forward horror, or western, whatever--crowd pleaser stuff, do ya. Basically, hack work meant for mass consumption, does indeed require the streamlining hand of an editor to make it more palatable (or sellable). Nothing at all wrong with that.
Now take your ARTISTS; your trailblazing geniuses out to create new forms, new ideas, new methods in which to tell different types of stories and books; sometimes such writers should be left alone, and their vision undiluted by the hive mentality of modern publishing. I would list a few examples, but then I'd just get a buncha comments telling me what idiots these writers are, or pointless arguments over what is, or is not, art.
Heck, I dunno. A fella can drive himself insane with these kinda thoughts, heh heh.