Omission may contribute to the sound pattern of an utterance. In verse it can protect meter (وزن) and rhyme (قافية), and in patterned prose it may support rhymed cadence (سجع). Certain inherited idioms also conventionally use shortened forms.
أو محافظة على وزن، أو سجع، أو قافية
Or to preserve meter, rhymed prose, or rhyme.
These aims do not permit arbitrary ungrammatical or unintelligible deletions. An omitted subject still requires an intelligible basis. Likewise, following established usage means respecting a form actually found in the language; it does not license inventing ellipsis everywhere to imitate its surface shape.