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But I am really worried with a deeper issue.
A full-body MRI scan provides information, and supposedly with no harm (at least physically)
According to your arguments, the medical system doesn't currently know what to do with it to provide an overall benefit (Allow me to be skeptical, but let's roll with it)
So your attitude is to not collect the information in the first place. I assume you are thinking that because there are no immediate benefits and lots of cost
But I'm sure there are, at least, future benefits. Why not instead start learning from this new source of information to do good in the future?, like at least, collect it for future references / comparisons with future scans
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If you still think the benefits do not compensate the cost, then fine, that's a completely personal decision that should not be imposed to others.
Doctor's role is to communicate as accurately as possible the benefits/harm of an action, and let patients decide for themselves if the cost is worth it or not