Depending on the country you live in, warranty may be a given by law. In Europe you automatically receive 1 year warranty on most hardware items, if not 2 years. This is regardless of the fact you send in the warranty registration card.
Sending in the card allows the company to register people who have their products and allows people to claim warranty without a receipt.
In the EU it is even so that after the mandatory warranty period, you may expect a product to last a certain amount of time, provided that the product is properly used (no abuse).
But as said, it does depend on the applicable laws in your country.