If I can just describe the set and derive the connections from that, I'll be much happier than if I'm forced to describe all connections every time. I don't understand the need for undirected arcs in that case. Perhaps I just don't have any interest in using RDF when a simpler more focused solution is possible. Run a few large web sites, and I promise you'll feel differently. Take a complex hypertext where documents link to each other as components of different sets.
HR meets finance, etc.
Once you start building dense hypertexts, having to explicitly describe every graph becomes a time suck in itself. Being able to describe sets and leave the graph derivations to software makes life much simpler. I remain unconvinced that style sheets are an appropriate location for this processing, but I don't think this processing should fall on the document author, either.