John, I'm sure new tyres are better however I did 10 years of trackdays mostly on other people's old tyres (Matt's, Dave Jarman's associates, ebay etc) and had a great time. Yes new tyres are a know quantity but for having fun (rather than racing) old track tyres are far better than road tyres. The main problem I had was tread depth consideration, the tyres gripped really well in the dry but the forecast for many trackdays was rain and it gave me headaches choosing which to use.
Also if you are driving on the road back from a trackday and start on tyres with only 3mm you will be risking it.