I'm not sure that fulltime proofreaders even exist today. They certainly don't at smaller publications. Years ago a proofreader was a proofreader and nothing more. They didn't edit and they certainly didn't write.
The proofreader was eventually replaced by technology and editors.
But, in book publication, the editor position is now replaced by the writer's agent. This isn't true yet with all publishers, but basically speaking publishers now want the writer's agent to do the proofreading and editing. The in-house editors are now called "packagers."
(There is a dilemma here, of course, because you can hardly get an agent unless you've been published and you can hardly publish unless you have an agent.)
Electronic publishing changes everything again as "packaging" is not as difficult.
The blurring of roles hasn't helped writing standards in my opinion. True editors love to edit. They don't want to be packagers and the best ones don't want to be writers. Many young college English grads seek editing positions dreaming of working with good writers only to find themselves the slaves of packaging schedules.
True writers want to write. They can edit to some degree, but basically they want to write, they want to create.
In my experience in the book world, editors who want to be writers usually make for poor editors.
In the magazine world, where articles may run from only 500 to 3000 words, it is easier for a writer to be an editor or for an editor to also write.
Years ago the fulltime proofreader carried a lot of weight. He or she was a staunch perfectionist and writers and editors quaked when the proofreader left the back shop and entered the newsroom.
