There's an awful lot of value in a particular book-as-object that you lose in a copy, especially in pre-Gutenburg manuscripts:
- What script was it written in?
- How was it bound?
- What shape and size is it?
- Huge roomy margins or small tiny margins?
- Is it actually a palimpsest with another text underneath?
- Who owned it?
- How old is it?
And so on. If someone just writes down the text and burns the book, you've preserved the text but thrown away an awful lot about the people that produced and used the book.
A 3D scan might preserve 100% of aspect A, but 0% of aspect B, in which case it's exactly as bad as no scan if what you care about is aspect B.