Well there's a feeling behind it. There's more to cars than track/street. Here's a pretty good example.
And you're dead-on about go-karts and Formula 3 (or even 3000) being ideal for track. I'd even say that street legal track cars like the KTM Crossbow, Ariel Atom, Caterhams, etc. are a good investment if you're at that level of enthusiasm/budget.
I am within a short road trip of Atlanta Motorsports Park which was designed by a Formula 1 track designer, and they have a kart track too. I went there with some friends and we had a blast on the karts. It is definitely one of the purest (if not the purest) forms of driving.
But go-karts aren't fun to look at. You wouldn't take a road trip in them. I find Formula cars fun to look at but I'm not on that level. I wouldn't want to take a street legal track car on a road trip either. And aside from the Atom, I couldn't take my girlfriend places or let her drive while I ride along.
A street car (I'll reference a VW Golf TDi, due to efficiency and practicality) doesn't quite stir the emotions as much for someone with that level of budget/enthusiasm. Nor is it as fun to look at. But a Supercar is
Plus Supercars are there for technology from race cars to trickle down into regular road cars. DSG gearbox, all wheel drive, hybrid/diesel, flywheel hybrid, regenerative breaking, etc. are all coming from Rally and Formula cars. Right now, we've got 3 main Supercar makers working on introducing flywheel hybrid technology to production cars. Ferrari with their Enzo successor, Porsche with the 918, and McLaren with the P1.
Maybe a few years after that it'll trickle down into the 200K range. And shortly after that I imagine the tech will be widespread. Regenerative breaking is too amazing a technology to not see everywhere.
