I disagree that this is a new trend, in my lifetime(born 1985) the big international road races tend to have been dominated by British Championship riders, I think when Michael Rutter won the 1998 Junior TT that was the same year he got on the podium in a World Superbike meeting, in fact in 1999 he did the TT while competing full time as a 500GP rider(the last to do so I believe).

Again look through the years at Hizzy, Foggy, Moodie, Simpson, DJ, Hutchy, McGuinness, Gaz Johnson, Plater and Duffus, these guys in their best years always had full time British Championship rides, Michael just needs to look at his own fathers achievements, Robert was a fantastic short circuit rider in his heyday. In the Sidecar class it's no coincidence that the dominant crew at the TT in recent years also happen to be the world champions on the short circuits.

I've heard many great road racers say that doing British Championship events(even if it's just a handful per year) really brought their riding on to a different level, look no further than Dean Harrison, for several years he was a pure road racer but since joining the Silicone Engineering Kawasaki team he's done a fair amount of BSB and Superstock events and his performance has shot up.