The thing about SIARPC is (and feel free to disagree) it's a segment that Clarkson designed to help attract mainstream ratings while at the same time make it one of the most off-beat parts of the British talk-show circuit.
On Jonathan Ross or Graham Norton, the celebs promote their latest projects and banter among each other. SIARPC offers a chance to deviate from their normal PR script and show that they're not afraid to fail onscreen. (The potential to reach that worldwide audience doesn't hurt either.)
Yes, of course, the celebs who have a genuine interest in cars and driving will do very well and relax, but even those with very little interest can offer some insight (Ross Noble bantering about motorbikes with RH is a real treat, if only to see JC's hanging his head in horror; Ron Howard's talking about Drive shows how he hoped to relate Formula One to a non-racing audience).
So of course Chris is going to keep it, interviewing guests is something he's learned to do and it helps attract mainstream attention to the show.