Too often I have seen Anselm's "ontological" argument abstracted from its context, as though the fact that his Proslogion begins with a prayer were inconsequential to the argument; or Descartes' proofs abstracted from his Meditations, as though it were not important that "God" serves an instrumental purpose for Descartes, allowing for the re-establishment of the world after he doubts its existence.
Anselm already believes when he writes his argument. He has arrived at his belief in some way other than argumentation, and there is no shame in that. Most of us arrive at most of our beliefs in less-than-purely-rational ways, and as William James has argued, we have the right to do so. It looks to me like Anselm is writing not in order to defeat all atheism (though that may be one of his aims) but in order to see if his faith and his understanding can be in agreement with one another.
Descartes might believe or he might not; I don't know how I could know. God matters in his Meditations because God offers an "Archimedean point," a fulcrum on which to rest the lever of reason, allowing Descartes to lift the world anew from the ruins of doubt. Whether or not Descartes believes in God's existence, God is useful to Descartes.
My point is that it is mistaken to assume that arguments about God - for or against God - are detached and detachable from other concerns, and when we neglect those concerns we might just be missing the most important aspect of those arguments, namely the human aspect. When we argue about God, we are usually also arguing about something else.