Question: When one has a good relationship with an editor (i.e. trusts his or her judgment) and is willing to undergo substantive edits, how necessary are beta readers? If they're still relevant, when would you use them? Do authors who work with traditional publishers use them?
I charge a flat rate for my substantive editing, but a lot of editors charge a range or hourly for that service, so if the editor has more work to do, then it would mean paying a higher price for that round of editing. Price aside, the cleaner the manuscript is before an editor starts work, the editor will better be able to hone in on the remaining problem areas without their attention and thought-process being divided between quite as many issues. I do charge a range for my line editing, and a beta reader could improve the writing at that level and thus save an author money there too, but I think beta readers generally have a lesser impact on that level of work, unless they really rock. :)
Despite all that, though, there are plenty of writers, traditional and self-published, that never have beta readers.