It is kinda ironic, that in the past, everyone rode, and raced on a combination of dirt and pavement. Same with auto racing -- the great old road races (eg Targa Floria, Mille Migla, PanMexican Roadrace etc) were run on both dirt and asphalt.
Over time, the sports became more and more specialized, but now we see somewhat of a reaction to that specialization. The fastest growing area of 4-wheel racing is probably WRC and similar national series, where rally cars race on a combination of dirt and pavement, and on 2 wheels, we have Supermotard making a comeback, along with, at least at the club level, competitive rally events for GS type bikes.
The specific bike platform used in Supermotard (Dirt bike with street wheels/tires and big front brake) is also not new at all. I know that in Colorado at least by the mid-70s, one of the most popular classes in the MRA (the local club racing organization) was for dirt bikes converted to roadracers. These bikes (either 2 or 4 stroke) frequently took the open class win.
Merlin Plumlee, who was Nicky Hayden's, and now is BBoz's, crew cheif was one of the pioneers in developing the XT500 as the dominating platform in late 70s MRA racing. They still run this class (called the "Colorado Class" in the MRA, although they have expanded the eligible bikes somewhat).
In the broader marketplace, I think the pendulum may be swinging back from the level of specialization of recent years, with bikes that are suitable for a wider range of riding situations.