I have rec'd such a memo from a company.  Speaking to others who do work for the co., none told me they would approve.  I emailed the person who sent me the memo, saying at this time I would not be adding them as "additional insured" and that I thought my E & O certificate was sufficient for their purposes...to please explain why they felt they needed to be added to my insurance.  I also emailed my insurer, inquiring about this, as I also have never rec'd such a request in all my yrs. of abstracting and doing work for numerous companies.  Well, I've heard from neither so far.  (Well, it's only been a day?!)  Robert's comment makes sense to me and so does my own common sense, so unless I'm convinced otherwise, I am not adding a client to my E & O!  That seems over the top!