The question boils down to "what is the actual advantage when 87% of the races from 1977-2006 were dry races?" In this period there have been 11 seasons where the title was decided by 8 points or less; only 4 of them are seasons in which the driver that scored the most points in wet races won a title. Those seasons are 1983, 1988, 1990 and 1999. In the 2007, we had 3 wet races and the title was decided by 1 point. The driver that scored the most points in wet conditions was Fernando Alonso and he technically finished 3rd in the title chase behind Hamilton and Raikkonen, drivers that have also demonstrated to be quick in wet conditions. However, if a rain master like Alonso can score the most points in rain conditions but lose the title, what difference does it make?
Given the data that was accumulated, wet weather driving ability is not so important. It is not important to the same degree that a single driver's car development skills are not important. If 87% of races in the past 30 years are dry races than the likelihood of using such skills is remote; in the same way that if a driver with poor car development skills is supported by good testers and a teammate that possesses such skills or there is a rules package that deminishes the value of such skills, the hinderance is minimized. In both wet weather driving and car development, the 'weakness' is hidden. In the first case, by the lack of wet weather. In the second case, by the skills of teammates or rules package. In my opinion you can be an average driver in the wet, maybe even below average, like Massa, and still be a successful driver.
There will be moments in time and history where these skills will be the difference. The period of 1987-1996, nearly 17% of the races were wet races (more than the average) is a period for driver like Senna to shine. In 2007, wet weather driving skills, or lack thereof, continued to not have an impact. However in 2008, we will see if it plays role. The 2 wet races this season have been won by Lewis Hamilton. He has a 6 point lead over Felipe Massa; again a driver that many consider to be below average in wet conditions. If Massa wins the title, it would continue to confirm that wet weather driving skills are fun to watch but not important to being a World Champion.
Data using FORIX: 1977-2006