Seems to be more common than it should.  See my post from 2009 on the subject.  I would never add a client as an "additional insured."

However, what they most like mean is that they want to be added as a "certificate holder."  This provides them with proof of your coverage and they will be notified if your policy lapses or is canceled. 

This should be a rather common request.  There has been some mention of abstractors who request E&O insurance and finance the premiums to get their policy, then they do not make the payments and the policy is canceled.  Thus, they are showing their clients that they have coverage but it is canceled shortly thereafter and those clients are none-the-wiser.

My guess is your client asking to be added as an additional insured just doesn't understand the terminology. 

Best,
Robert A. Franco
SOURCE OF TITLE