You misunderstand. Given that data MAY be a graph structure, the markup MUST support that. A tree with pointers to the parent nodes has a graph structure, a common enough occurrence.
> a common enough occurrence
Do you have a concrete figure about "common enough"? Otherwise we are only talking through anecdotal evidences (not necessarily bad though!). At least in my experience, graph structures do occur from time to time but almost all of them can be easily rewritten to non-graph form because they are typically DAGs and obviously ranked (i.e. its preferred topological order forms partitions of different nodes). The general graph that requires something like references was very rare.