Cyonic preservation is an ethical way to treat a terminal patient provided the patient himself requests it. This is a new form of "treatment" for terminal cases, and it still gives hope to people that they may one day come back, be cured, and have a new life instead of the disease they had before.
It is true that currently it is essentially impossible to thaw a human that was cryonically frozen and bring them back to life even if they were perfectly healthy. That is not to say that it could not be a possibility in the future. If a patient wants to choose to subject themselves to that gamble that should be their choice.
I think that it's perfectly acceptable. However remote the chance, there is some slight possibility that we can bring people back although with current preservation methods that is highly unlikely since there is so much damage to cells. But people are free to choose how they want their body disposed of so long as they pay for it.
I think the idea of freezing somebody who has a terminal sickness is an unethical way to help cure somebody. I do not agree with the idea. But I also believe that people should have the right to decide what they want to do with their treatment even if that is an option.
Not at all who wants to preserve anything of themselves if they have been ill and who really wants to come back to life if at all after they die. I mean our loved ones might want to keep us alive forever, but no I wouldn't personally want to come back to life after dying.