Adams says that a Christian ethical view needs to allow for supererogation to capture the sense of options in Christian life. There's no other way to account for Paul's insistence that Christians are free in Christ and no longer slaves, that Christians are friends of God and no longer in servitude. I have two responses, one exegetical and the other philosophical.
The exegetical point is that I think he misconstrues Paul's point. Paul isn't saying that we are free from God's command. The freedom is first of all a freedom from sin. It's a freedom to serve God, which is put in slavery language. Christians are no longer enslaved to sin but are instead enslaved to God. This picks up on the language of Exodus. The people of Israel were freedom from slavery to Pharoah to become slaves of God. The Hebrew term in question is often translated as "worship", and so translations often say that Israel is freed from slavery to Pharoah to go worship God. But the verb is the same. It's a movement from slavery to Pharaoh to slavery to God. God is the master. It's just that God is a master who loves his people and wants what's best for them, while Pharaoh is just taking advantage of them.
The parallel language in Paul's epistles about Christians being freed slavery from sin to become enslaved to God should be no surprise given the old covenant antecedent. Freedom in Christ is slavery to God. So I don't see how the movement from slavery to freedom involves moral permissibility to do as we wish provided that we meet some minimal moral threshold. It in fact binds Christians to serve God fully and completely, to surrender any self-directed goal in favor of becoming like God, having a heart that values what God values, having motivations that line up with God's will, and acting in a way that a morally perfect being would act. This is in fact what the Sermon on the Mount enjoins. "Be perfect, as your heavenly Father is perfect", an echo of Leviticus 18, which says "Be holy, as I am holy."
Now this doesn't mean that there aren't options in Christian living. As Adams points out, there are two ways of generating options. One way is supererogation, which allows for the less-than-perfect to be morally permissible. That's what I don't see Jesus allowing for. The other way is what Adams calls indifferent actions. These are things that are equally good, and so we have the option of choosing whichever of the equally good things we will do. If there really are equally good things, all things considered, then I have no problem with those.
I'm not sure they will easily occur, though, and Adams seems to agree. He just says it's because of nuances in ethical importance that may play a role. I can imagine he has in mind things like the fact that two actions might be equally good but that one of them involves going against my natural tendency and thus allows me to develop a trait that I ought to work on. He might have in mind two actions that, other things being equal, are equally good, but one of them involves a better fit with my special obligations to my family. In such cases, it's pretty clear to me that the one that is better, all things considered, is morally obligatory. So these aren't options after all. But there is room for all considerations to work out equally. It just doesn't seem likely that they will be exactly equal. What seems more likely is that they will be so close to equal that I won't be able to discern the moral difference or the balancing out of moral considerations in the right direction. There is always the problem of figuring out what is the best option when various possible courses of action appear in front of me.
This difficulty suggests to me a philosophical distinction that I think lies behind my disagreement with Adams. He wants a moral theory that allows for options in order to explain the difference between legalism and Christian freedom. But he is locating that difference in moral obligation. There can't be moral obligations that I ought to do, or I am not free in some sense. I am not morally free to do what I want. I think this is the wrong place to locate Christian freedom, because I think we do have an obligation to do what is best. It is a moral obligation, not some other kind of constraint. What Christian freedom amounts to is not freedom from moral obligation. Paul even says so. He says there's the law of Christ.
What we don't have are very specific laws that are to be followed absolutely, without room for reflection on whether those laws apply in our case or whether those laws conflict with other laws and what we should then do. Christian freedom, on my view, consists of not being bound by laws to be followed without reflection. It consists of being bound by general moral principles that require careful thought about what we ought to do, what we ought to be motivated by, what attitudes we ought to have, what character traits we ought to be developing, and so on. Adams seems to want freedom from obligation, but I think Christian freedom is rather freedom from rigid rules. Morality isn't about rules. It's about conformity to a standard, a standard who is a person. Christian morality has to do with being conformed to the image of Christ, being transformed to becoming perfect. It is much more complete than simply an ethics of action. There is something morally wrong about us if we are not perfect, and our moral obligation is to pursue perfection. This is the thrust of the ethical teaching of Jesus and Paul both (along with the rest of the Bible, I might add).