Just looking at figures from Snetterton
 

DD (2 Classes) No Wets
Race 10 - Starters = 23, Finishers = 18, DNFs = 5
Race 14 - Starters = 14, Finishers = 7, DNFs = 7

Modern Classics (5 Classes) allowed wets
Race 11 - Starters = 19, Finishers = 17, DNFs = 2
Race 15 - Starters - 12, Finishers = 6, DNFs = 6

But what does it all mean Kev?

It means your just as likely to have only half the starting grid finish even when your allowed Wets