From one of the tech articles on F1Technical:
2007: Contrary to previous years, all cars are required to race both available tyre compounds sometime during the race. To mark the difference, Bridgestone came up with an idea to paint one of the grooves white when it consists of a soft compound (the softer one of two dry weather tyre compounds available) or the full wets (compared to no marking on the intermediate tyres).
Who is responsible for this requirement? There are obvious safety issues involved in racing with two levels of performance in tires. Also, throughout the entire history of racing it has been the goal to optimize the performance of the vehicle in order to gain victory. This rule deliberately compromises the performance of the car and requires the driver, in the heat of battle so to speak, to instantly adapt to altered traction levels in his or his competitors vehicles.
I am amazed that the FIA, and the drivers in particular, accept such an unsound practice. Reading the reports from the free practices in Australia a large number of drivers and engineers commented on the problems of adapting to tires, graining, not getting up to temperatures and what have you. I can't image that Bridgestone thinks this is good advertising for their products.
So, does anyone out there know why the tech rules are this way?