Desirism holds that there is no intrinsic value - that the only way to determine the value of one desire is by its contribution in fulfilling or thwarting other desires.
One of the implications of this is that, in a simple community with two conflicting desires, desirism may not be able to have anything to say over which should win.
Let us assume that Person 1 has a desire to kill Person 2. At the same time, Person 2 wants to work in his garden. These are the only two beings in the universe. Can desirism tell us the moral value of Person 1 killing Person 2? The question is ambiguous. Because of this ambiguity, there are two potential answers. In one sense, there is no answer. Moral concepts do not apply to this situation. In another sense, it us wrong for Person 1 to kill Person 2.
In the first sense, given that Person 1 has a desire to kill Person 2, and the fact that he will act to objectively satisfy the most and strongest of his desires (given his beliefs), he will be trying to kill Person 2. That's just a simple fact of the matter. Person 2 wants to work in his garden. He cannot work in his garden if he is dead. Therefore, he has a motivating reason to avoid a state of being dead. His desire to work in his garden generates behavior that aims to thwart Person 1's desire to kill him.
These two are locked in conflict.
Even if both desires are malleable, there is no sense in which morality can find them a way out.
If we assume malleable desires, then Person 1 has a motivating reason to rid Person 2 of the desire to garden (so that Person 2 will quit trying to protect himself). Person 2 has a reason to rid Person 1 of the desire to kill him. In this case, Person 2 may decide to try to kill Person 1 first. Killing a person has been proven to be an effective way of altering their desires. There still is no moral argument to be made in favor of one desire or the other. However, when we look into that universe and make an evaluation, we are not within that environment. We look in from an environment with seven billion people who may or may not kill us. We have reason to worry - consciously or unconsciously - about the implications to our own survival of different attitudes towards the conflict between Person 1 and Person 2.
We have good reason to prefer to be surrounded by people who look into this world and find the person with the desire to kill repulsive. We have reason to encourage that attitude - to promote in others an aversion a disposition to condemn those with a desire to kill as well as an aversion to killing.
One of the ways we can promote this aversion by praising those with an averse reaction to killing and condemning those who are indifferent to the two desires. The language we use for praise and condemnation is moral language. Calling something permissible tells people to have an attitude of indifference to the available options. Calling something bad and another good tells people to have an aversion to the first option and a desire for the second.
In that moral language, telling people that it is not wrong to kill means telling them that there is no moral difference between the two options is to tell them to be indifferent to the desire to kill - that they should look upon the desire to kill and shrug their shoulders with indifference. We have many and strong reasons NOT to say such a thing to people we have to interact with on a daily basis. Instead, we have many and strong reasons to tell others, "Have an aversion to the desire to kill. Be repulsed by it." We do this by saying that the person with the desire to kill is evil, that a good person would try to stop him and defend the gardener. In this sense, we look in on this imaginary world and tell our fellow observers that Person 1's desire to kill is wrong. It is true that, within that universe, moral terms do not apply. However, we do not speak within that universe. Our moral language is not heard by ears or interpreted by brains within that universe. We speak in this universe, and our words have real-world effects on real-world brains that we have many and strong reasons to consider carefully.
In this universe, moral terms have implications for the attitudes that the people around us adopt. They have real-world implications affecting how likely it is that real people will be killed. Because of this, we have real-world reasons to tell others in our community to have an aversion to the person with the desire to kill. Calling Person 1's desire to kill Person 2 "wrong" is not a mistake. It is not an illusion caused by the way we have trained our attitudes in this world. It is a fact. The person who calls the attitude "wrong" is making a true statement. However, that true statement is not, "There are properties in that universe that make the desire to kill another wrong." The true statement is, "People in this universe have many and strong reasons to promote an aversion to the desire to kill - which we can do by morally condemning those who have that desire, such as Person 1 in this hypothetical universe."