Our drywall installer tells us that we need to prime the walls before his finisher can fix the MANY issues with their finishing work so far. I can’t find any information that suggests this is normal practice. Is this guy screwing with us?
I'd say it's a little from column A and a little from column B.
A good drywaller will get the finish to a point of being pretty darn good before primer. A bright light shone sideways on the wall shows where work is needed.
That said, primer shows all the little (emphasis: they should be little) flaws that the painter usually touches up.
So, I'd suggest you try and get him to do his best before paint. If you're concerned about the job being finished, holding partial payment back is common. Don't be punitive, but don't make it such a trivial sum that he can write you off.