A grammatically complete relative clause does not automatically make an effective identifying expression. It must supply something the addressee can use to recognize the intended subject. A clause entirely unfamiliar to the listener may leave the reference obscure.
Contrast:
«الذي قابلناه أمس فقيه» — “The one we met yesterday is a jurist”: effective if that meeting is familiar.
«الذي رأيته في مكان مجهول فقيه» — an unknown meeting in an unknown setting may fail to identify the person to the audience.
This is an issue of rhetorical fitness, not simply syntax. The usefulness of the clause is measured against the listener's existing knowledge.