Do not confuse the economy of annexation with omission of the subject. Ellipsis leaves the subject unspoken and relies on a cue; annexation retains the subject but identifies it compactly through a relation.
Expression typeRhetorical operation
EllipsisSubject unexpressed; recoverable through context
Definiteness by iḍāfahSubject expressed, identified by its annexed partner
Definiteness by a relative clauseIdentification through the descriptive relative clause
When the purpose is to reveal a relationship, annexation can be concise without losing the identifying expression. The benefit is not simply fewer words but clear identification through a meaningful link.