i) Hypothetical scenarios are a fixture in ethics. But some Christians are leery of hypotheticals. They think these are used to confound or derange our moral intuitions. And hypotheticals can sometimes be misused in that respect.
ii) However, hypotheticals are both useful and indispensable. The Mosaic law contains many hypothetical cases. The parables of Jesus often present hypothetical situations.
Even people who express suspicion about hypothetical scenarios resort to hypothetical reasoning all the time. They deliberate on alternate courses of action. They speculate on the consequences of different choices. Mentally compare and contrast each outcome. Predict what's likely to happen in each case. That's a basic part of planning and decision-making. So it's unavoidable.
iii) One purpose of hypotheticals is to test moral consistency. Are you prepared to take your position to a logical extreme? Are there exceptions to your principle? If so, are those ad hoc exceptions?
iv) Although that's often a valid exercise, we need to be careful. Precisely because hypotheticals may be unrealistic, because they are not constrained by what's possible or probable, it is easy to generate false moral dilemmas.
But that's not necessarily a reason to question your principle. It depends on your view of divine providence. As one philosopher recently put it:
I think we should, however, take seriously the possibility that as we depart far enough from the normal operating conditions of human beings, some of the questions (a) have no answer or at least (b) have no answer available to us. This possibility undercuts some arguments.
For instance, one can argue that utilitarianism gives deeply implausible answers (e.g., that every action is equally permissible) in cases where there are infinitely many people. But suppose that there aren't in fact infinitely many people, and the situation of there being infinitely many people is far beyond humans' normal operating conditions. Then the fact that utilitarianism predicts something that seems implausible to us beyond those conditions is not a problem for the utilitarian--as long as she is willing to modestly limit the scope of ethics to humans in or near their normal operating conditions (if she's not, the argument is fair game).
Or consider this argument against deontology: It seems permissible to kill one innocent person to save a billion. But circumstances where we choose between one life and a billion lives might well be so far beyond our normal operating conditions that they fall beyond the scope of ethics.
The last case is interesting. For it raises this question: Might we not actually find ourselves in circumstances so far beyond our normal operating conditions that ethics doesn't apply…? After all, it is sadly all too easy to imagine how someone might end up choosing whether to kill one innocent to save a billion...It seems deeply troubling to suppose that some people end up in circumstances that go beyond the presuppositions in the moral law.
I think Christians have reason based on revelation to think this doesn't actually ever happen. The moral law is also embodied in revelation, and revelation presents itself as a guide to us in all the vicissitudes of life. But note that even if nobody ends up in circumstances that go beyond the presuppositions of the moral law, going beyond these presuppositions could be physically possible but for God's providential protection. A case of choosing whether to kill one innocent to save a billion may be like that: God makes sure we're not tried beyond the edge of ethics.
But is that pious hope consistent with freewill theism? Seems more consistent to say a person might find himself caught in an intractable moral dilemma–since God doesn't control whatever happens–in which case God will excuse him.
v) Finally, hypotheticals can be valuable in part because they may be far removed from what we've experienced thus far. And we may never experience something like that.
Thing is, you never know ahead of time what life may throw at you. Consider American soldiers who wound up on German or Japanese POW camps. Or consider wrenching medical decisions which may confront us some day.
A value of hypotheticals is to formulate a position before you find yourself in that position. You have the leisure to think about it, to weigh the evidence. To developed a considered position without the duress of a real life crisis.
If you wait until you find yourself in a dire situation which requires a snap judgment, that can be the very worst time to think about it for the first (and last) time. You have to make a momentous decision. You don't get a second chance.
Hypotheticals help us to work out some positions in advance, when our judgment isn't clouded by emotion. When we can consider the issue with a degree of detachment. When we have time to become informed. When no one is pressuring us to act in a certain way.
vi) Mind you, armchair analysis has its limitations. Because it's abstract, we may fail to anticipate some important considerations. Because there's no urgency, we may not give it in-depth consideration. It's just not that relevant at the time.
Experience can inform, refine, revise, or sometimes reverse our answers to hypothetical questions. But it's better to go into the situation with some intellectual preparation.