I just got spam for a proofreading service with the following blurb:
Please excuse us for contacting you directly. We are offering an efficient service to help you ensure your academic and professional work is written in perfect English. We will check the grammar and style of your work and return it to you to meet your requirements and deadlines.
Would I need someone to help me write perfect English in my work? Not that I write the epitome of perfect prose, but I would think that my writing skills are "good enough". What makes the use of such services even more dubious for academic work is that one often needs clarification from the original author on many things, sometimes even going into deep discussions about the topic, lest your editing for better English makes the text mean something completely different from what it's supposed to mean. I should know: it happens often enough to me when editing something written by a colleague. I can't imagine it being any better for a distant proofreading service with little opportunity for interaction.
Of course, such services might be useful if your writing skills are truly horrible, where you could fix the text in a much improved manuscript than what you originally wrote ...