Whilst I think on balance the scans should be public; it is more important that the FOI laws strike a perfect balance of enabling efficient administration whilst shinning enough light to reduce corruption.
I would not want museums to be reluctant to create scans and archives that compromise their short term funding models if they are useful for conservation.
However storing them idle in a locked filing cabinet for a conservationist to reference in 100 years may well still be valuable.
To say they are useful for conservation is to say it's for the public, there is afaics no separation of concerns here unless you're arguing the museum should act a private collector in its conservation?
To counterpoint your next argument that the conservation using this material must be handled by specialists, yes it should, if they were casts of bones etc and are themselves genuinely rare and difficult to replicate.
Digital goods are only rare by virtue of the body holding them not making copies... By acting in a private capacity.
More important than what? What is a perfect balance?
This is not a good way of stating an opinion about what one should value.