I have not had many such clients; I believe that most businesses simply don't have time. But if you are dealing with universities or researchers, in my experience you'd better get ready to defend perfectly good English.
A few years back, I had a bad experience with a German research institute. They criticized several things on each page over the entire 25-page report. I told them to put up or shut up: they were to find an independent expert/translator to review my work, and I would pay for that external expert if the review turned out to be negative. If the review was positive, they would pay me immediately.
The reviewer found a single mistake those 25 pages: I had put "tonic" instead of "toxic" somewhere. The research institute was humbled.
I have not worked for that client again since (the contact was over a translation agency anyway, and I no longer work for agencies), but the experience would certainly have made me look better in the eyes of that client.
Right now, I am trying to educate another of my clients, and the process is tedious. Last month, the client wrote in and asked whether things like "ABC used to be used for XYZ" could not be reworded (the apparent double use of "used" must have been the problem, though the two words are different parts of speech here and actually even have a different pronunciation, with the -s being voiceless in the first word and voiced in the second, at least in my pronunciation). This month, the "corrections" I received included things like:
- "The figure has to be recalculated again" (I had left out the word "again")
- "Mainz at the Rhine" (I had referred to the city simply as Mainz, but notice the preposition -- nonetheless, "Mainz on the Rhine" does not exactly clear up any confusion, for there was none)
- "standard deviance" (for standard deviation)
Having said that, I also got some really nonsensical edits from the publisher of my book "Energy Switch" (but that would be a different blog post), so I suppose it really comes down to the person you're dealing with.