Partially disagree
If the Value formula of a computed field
returns @Unavailable (or @DeleteField, for that matter), then the corresponding
item will in fact not be present on the on-disk document after
you save the document. The same is true after running ComputeWithForm against
the document -- as described in the OP's use case. So, a Readers field
formula that returns @Unavailable will not leave the document unreadable.
Apart from that I totally agree with
you that it is good practice to always have Readers fields contain at least
a role that is reserved for admins (plus optionally one reserved for servers).