An authenticated user with maySetKeywords on another user's mailbox could change that mailbox's specialuse annotation.  This could allow the sharee to change the shared mailbox to perform the archived, snoozed, or other role, which might cause mail mail to be written to the shared mailbox, sharing more content than intended.

This is likely to be an unusual situation, made more unusual because if the target already has an non-shared mailbox with that role, role duplication suppression will prevent the update.