The thing to remember on track days is if a car is behind you and wasn't there a few corners ago, he's clearly faster than you so you might as well let them go, regardless if you are quicker in the corners.
What I like about about a slower car is its easier and quicker to reach the limit than in a stupid quick car. I can for example, get to drive my Elise and DC5 at 80% far quicker than say if I had a 911 GT3. It would take many years, many miles on track and tuition to even get to 80% of what the car is capable of. To be honest, unless I am a driving god (unlikley), I don't think I would ever get to the stage where I could drive a 911 GT3 at 80% of its limit. An example is when I took my wife's 1.6 Mk1 MX5 on track at Brands. It was pathetically slow with everything passing me at first. After some tuition and shown how to exploit the power and handling, I was now being held up by Renaultsport Clio's, Porsche's and a 350z. Still being killed on the straights but drifting through Paddock Hill was such a great feeling. The point is, I was able to reach a higher lever much quicker due to the slower nature of the car and easier learning curve. Add to this the benifit of tuition and my own personal experience, within an hour I was having a hoot and lapping as quickly as I had previously in faster cars.
The other thing worth rememebering is some people want to go fast on track and some people want to be a quick driver. Both are two very different things. Some people are more than happy to turn up in a crazy quick M3 and bomb down the straights and have no interest in maximising corner speed, balance, braking etc. as they are just happy overtaking cars on the straights. Other's turn up in little Renaultsport Clio's and want to learn how to drive fast on track and the only way you can do this is getting mileage and tuition under your belt.
As has been mentioned already, the more you do something the better you get and this is equally valid on track driving. This is why getting up to speed in a slower car is quicker, easier and has long term benifits when starting out rather than a crazy quick car. For the last two years I have been heavily tracking my DC5, roughly doing a track day a month including Spa. My focus was to learn how to drive the car quickly. I've been with the same Lotus and Honda group throughout so I've had a good gauge to measure my progress. When I started yes, I was slow and struggled to keep up but now with lots of track miles under my belt and tuition, I'm either keeping up or passing them and certainly in the wet (my favourite time on track) I'm one of the quickest. Two years on, I'm now being invited to track days with the VTEC Challenge race drivers. Why I asked, I was told I'm one of the quick guys now and at Combe last week at their official test day, in the dry I wasn't being lapped and in the wet I was pretty much keeping up. So practise does make perfect but more importantly I honestly believe that if I started out in a really quick car, I would still be coming to terms with it now, certainly not exploiting as much from it or near its maximum as I am the DC5 and certainly not having as much fun and enjoyment. It's also worth mentioning that not only is it important to learn the driving characteristics of a car, its important to understand the car itself, learn what works on it, what doesn't, what to change and tweak and what to leave well alone. Understanding the car, its characteristics and how to drive it are all part of the big picture.
As for a Caterham, I've only driven a 1.6 in anger around Silverstone and Brands with one of the Lotus instuructors next to me and its probably the most fun car I have ever driven. So much feedback and so rewarding. Truely amazing cars so totally agree with the comments everyone has made.