Flexibility is also (at least for now) the key reason to look at an RDF storage engine.
I guess from a relational database perspective these are just "Entity-Attribute-Value tables" except that of course the attributes and values are also (potentially) entities...
If you actually have a schema that reflects your system's unchanging reality, an RDBMS remains an excellent choice, with lots of mature offerings, excellent performance, etc. RDF will not ever be a competitor for the certainty of "Every X has exactly one Y, which is a unique integer" if you can bludgeon reality to fit such rules.
The interest in key-value stores suggests that more and more people are realising that their problem isn't entirely amenable to this approach. But I'd argue that key-value is not quite flexible enough either.
But it could be that I've been using a hammer so long that now all the more sophisticated problems just look like they haven't been hit hard enough or from the right angle.