I'd argue from practical experience that an SQL which universally enforced set-relations would introduce significant implementation overhead: DISTINCT on large

I'd argue from practical experience that an SQL which universally enforced set-relations would introduce significant implementation overhead: DISTINCT on large relations is v. expensive! I believe it would also add complexity on net for users, as they would have to re-introduce those "hidden identifiers / row keys" in the not-uncommon cases where bag semantics are desired.