i can se your point there. i understand what your saying but i think you may be not seeing why a stig is needed. he is an unbiased and unknown driver. that makes his lap times more "relevent". if, for example, you knew the stig was Jeff Gordon and you know he is loyal to GM and lets say that he was gonna run lap times in a Vette and a Mustang, well you know GM would expect him to make the vette shine. having a secret person do these things is very good for the show. the public (for some reason) fels that they can trust him/her/it more than the host who will make thier alegence known for what brand they suport more.
a good example of this is the Porsche vs Ferrari argument between Jezza and Richard. you know jeremy is going to push the ferrari a little harder. putting the stig in the cars for lap times settles the argument. remember, he/she/it doesnt have a preference so its a fair way to test the argument. think like this, if you and a friend were arguing over what soup was better and you just couldnt agree, the best way to solve it is to ask a third person and even better if you dont know the person or even better than that if the person was masked so you couldnt see thier face and you handed the soup to that person, well, the answer they gave, as far as you know would be unbiased. you can just trust that more and your argument would at the very least have a result of some sort.