Editing and Macmillan
My own first novel (On Bended Knees) came out from Macmillan. Editing was a brief and happy affair (I was flying off for months in Thailand which encouraged agreement, waving a rapid goodbye to a favourite chapter).
With my four books since then, editing has been minimal. I've even wondered whether the editors actually read the books - their one or two suggestions seemed more intended to stamp authority on a book than working on the book's behalf.
Copy editors have gone to town on them - how wearying that can be. Several libel lawyers combed every sentence of In Search of the Divine Mother. Editors fight to get the book through their systems, they get the blurbs and the cover art and the support of the house, but in my own experience the impact of their editing has been pretty minimal.
As Ben Ball said (see the posting below this), while he always expects to have some input to a book, manuscripts are ideally polished to the extent that they are ready to fly when they reach an editor's desk. Personally I am quite prepared to pay someone to do that job, to bring in an experienced, professional objective opinion, and have done so in the past. An alternative is to rope in a writer friend, which I have also done, and trading services is possible there (why should writer friends work for free? And are friends the mnost objective readers?)
I can see, as one of those posting comments stated, that one does not wish to pay the publisher for editorial work on one's own book. (I'm surprised the contract of my own biography sees me paying for any index, which strikes me as highly dubious.) But I can also see the point of bringing in outside editorial assistance so that a book needs very little work when it reaches an editor.