Yes I agree. I know for a fact that McKinsey asks for GMAT (or LSAT or other test scores). I don't know if they are absolutely necessary for employment, but I do know that they ask for it.
I'm also quite certain that business schools are aware of their customers (the companies that hire their graduates), and that it is important for them to keep the scores of their admitted students high because they will be more competitive in the job market.
In fact, I will paraphrase from the University of Chicago message board, a message from a GSB student to an applicant with a low GMAT score: It would be wise to try to improve your score because even if you are able to get in, your lower GMAT score will continue to haunt you as you look for a job.
So even for a top top school like Chicago, employment may be more challenging for students that do not have top GMAT scores.