The credentials simply add to the marketing aspect in that you can say that you have them and that (hopefully) that credential comes with the ability for you to say you have completed specific training in this area- it gives you something to use to market yourself in terms of what knowledge you have for those tasks.
I think some clients like to see that and others don't really care. Having a credential might get the client to call you but really they are going to know by how you present yourself and how well you speak to the topics they are interested in as to whether or not they feel you have the experience they seek- which I think is perhaps also part of the point that Serena was trying to make (Serena, please correct me if I'm wrong about that
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