I think announcing Schumacher as the Stig was just a mixture of a joke and also as an appropriate way to introduce one of the greatest F1 drivers the world has ever seen. Just saying 'Ooh and he's Michael Schumacher' would seem a bit bland. Pretending he was the Stig gave it a bit of flair.
Personally I'm glad they gave it away at the end, I don't want to know who the Stig(s) are. It makes all those jokes pointless and they wouldn't be funny if you KNOW its a real person. The fantasy is far better than the truth.