Continuing on from my statement that God is only necessary if he exists, I see Kant has talked about the very criticism, and is, in my opinion, absolutely spot on.
Here is what he had to say:
"Immanuel Kant put forward an influential objection to the ontological argument in his Critique of Pure Reason. The critique was primarily and explicitly directed at Descartes, but also attacked Leibniz. Kant's refutation consists of several separate but interrelated arguments, shaped by his central distinction between analytic and synthetic judgments. In an analytic judgment, the predicate expresses something that is already contained within a concept and is therefore a tautology; in a synthetic judgment, the predicate, or claim, links the concept to something outside it that is not already logically implied by it. New knowledge consists of synthetic judgments.
Kant questioned the intelligibility of the concept of a necessary being. He considered examples of necessary propositions, such as "a triangle has three angles", and rejected the transfer of this logic to the existence of God. First, he argued that such necessary propositions are necessarily true only if such a being exists: If a triangle exists, it must have three angles. The necessary proposition, he argued, does not make the existence of a triangle necessary. Thus, he argued that, if the proposition "X exists" is posited, it would follow that, if X exists, it exists necessarily; this does not mean that X exists in reality. Second, he argued that contradictions arise only when the subject and predicate are maintained and, therefore, a judgement of non-existence cannot be a contradiction, as it denies the predicate.
Kant then proposed that the statement "God exists" must be analytic or synthetic—the predicate must be inside or outside of the subject, respectively. If the proposition is analytic, as the ontological argument takes it to be, then the statement would be true only because of the meaning given to the words. Kant claimed that this is merely a tautology and cannot say anything about reality. However, if the statement is synthetic, the ontological argument does not work, as the existence of God is not contained within the definition of God (and, as such, evidence for God would need to be found).
Kant goes on to write, "'being' is obviously not a real predicate"  and cannot be part of the concept of something. He proposed that existence is not a predicate, or quality. This is because existence does not add to the essence of a being, but merely indicates its occurrence in reality. He stated that by taking the subject of God with all its predicates and then asserting that God exists, "I add no new predicate to the conception of God". He argued that the ontological argument works only if existence is a predicate; if this is not so, then it is conceivable for a completely perfect being to not exist, thus defeating the ontological argument."
"All of our behavior can be traced to biological events about which we have no conscious knowledge: this has always suggested that free will is an illusion."
- Sam Harris