There's another implied argument, that a DB should be relational for the sake of being relational, instead of due to some other reason.
If you're doing aggregation or relational queries then yes, a relational database makes sense (unless OLAP makes more sense, but whatever ;-) However, in an OLTP context, a relational schema might not offer clear benefits over something else, and in general people have a habit of using them anyway, and working really really hard to use them too, without having an actual reason except that they're used to it. Just because you *can* represent a graph as 3-tuples (RDF style) or a table of vertices and a table of edges, doesn't mean that you should. There may be easier ways to traverse and work on a graph, depending on what you actually need to do.
