I see both sides of the page on this one.
A writer pours so much of them-self into a piece of work that sometimes it is difficult to see it stripped back and rearranged.
Editors are perfectionists who see the potential and want to guide a work to be better and achieve all it can be.
Personally I'd tend to think that it depends on the relationship between the writer and the editor. Some people are only wanting someone to proof read their work and check for spelling and inconsistencies. Others seek for their efforts to be shaped and guided. Yet others dream of seeing their work fly high with the work of an editor.
Don't forget there is a variety of editing styles. If your editing to publish I'd completely agree that you need to be harsh. The success of the work depends on it. If a writer is writing for enjoyment and as a hobby is it necessary to be as harsh? I don't know, however I do think that there is a balance that could be struck by agreeing the involvement of the editor from the onset.
Lets be honest, an editor is key to style, sense and continuity of a story. As an editor albeit for script work, I appreciate that my own written work needs its own editor in order to be all it can be. Even my own writing needs editing, I too make mistakes.
Taking a raw work from its first draft to its completed presentation is indeed the work of a team effort from the writer, beta readers, and editors too boot. Personally if I read a story that grips me I can read beyond the errors, even if I have to bite my tongue, but I've also frequently stopped reading a work because it is just too frustratingly poorly written.
I've read some brilliant points by editors above this post on this thread and agree with many. I also understand the flip side of that coin though, so guess I'd say everyone is different in their approach, the way they work and the results they achieve.