Ah I see.
I do, yes, based on the principle that I cannot afford to replace the car if I write it off. Well, I did on the Zed and the 911, but I don't with the MX5 'cos it's not worth a whole lot!
It's risk-based: If you're happy with your own driving AND feel lucky that no-one else will drive into you, or can afford to replace the car if you bin it, then don't bother. If you feel nervous about any of that, then get some insurance. You'll be better off getting a road policy that includes trackday coverage, but for the odd day here or there then just get single day cover.
Moris will quote you for the Ring, and they're quite reasonable too.