This is a very interesting argument. I think the primary strength of the future of value argument is that it helps explain the wrongness of killing unconscious people (including embryos) or suicidal people, which utilitarian or desires-based arguments can't really. The question regarding whether it would be wrong to smother the terminally ill cancer patient is best answered in terms of human dignity, however. Clearly, the patient isn't being deprived of much future life, but they certainly have a right to die in peace, rather than violently, and of course this is generally why it is very easy to see why a violent murder (including any abortion where the fetus is obviously conscious) is wrong. The weakness of the future of value argument would be that it does seem that there would be a subjective valuation of someone's future. Probably the rebuttal to this objection would be that we don't infallibly know whether someone's future will be valuable to them or not, so they ought to be allowed their chance at life anyway. I can kind of see how the future of value argument need not be the first one to be used from a Christian perspective, how the Image of God and the human dignity that follow are the primary reasons for the wrongness of killing; however, such an argument can easily be couched in Christian terms, i.e. that God has a plan for their life. God is the only one who has the right to give or take away a human being's "future of value;" it is not a right given to us to violate His will for someone else's life. The follow-up to this line of reasoning, which secular people will not agree with, is the wrongness of contraception. If God had planned for a particular sexual encounter to result in the creation of a new human being, it is not our right to violate His will in this regard. Obviously, there is no particular person being killed here, but it is still wrong nonetheless, and according to certain Church Fathers contraception is practically murderous in intent even if not actually murder (and is mortally sinful for a variety of other reasons).
I can kind of see how the future of value argument need not be the first one to be used from a Christian perspective, how the Image of God and the human dignity that follow are the primary reasons for the wrongness of killing; however, such an argument can easily be couched in Christian terms, i.e. that God has a plan for their life. God is the only one who has the right to give or take away a human being's "future of value;" it is not a right given to us to violate His will for someone else's life. The follow-up to this line of reasoning, which secular people will not agree with, is the wrongness of contraception. If God had planned for a particular sexual encounter to result in the creation of a new human being, it is not our right to violate His will in this regard. Obviously, there is no particular person being killed here, but it is still wrong nonetheless, and according to certain Church Fathers contraception is practically murderous in intent even if not actually murder (and is mortally sinful for a variety of other reasons).