They are absolutely perfect for lightweight purpose-built race-cars - in the Radicals we lap only fractionally under the much more sophisticated and better balanced FFord Zetecs yet the engine costs hundreds to re-build rather than thousands. It's hardly modified as it has the all the right characteristics for a RACE engine.
I find the idea of driving something like this on the road very appealing (just the noise is tempting enough) - I'm curious, though, in how they would perform in a regular road-going Caterham with a passenger in. Would such a car be useable in real-world conditions?
There are indeed some enduring prejudices in club racing which don't really stand up to argument but are nevertheless understandable - if you invest a lot of time and money converting and maintaining an engine originally designed for a day-to-day saloon, and then some upstart whips your ass with a stock engine half the size, it must hurt! However the latest 750MC Sports 2000 class shows that they can run side-by-side with "car"-engined racers and have a good battle.