Re: How great is Nadal on clay? Check it out!
Leading off from what don said, it would not be such a good idea to limit the amount of tournaments each player participates in each year/three month period. Tournament directors rely on big time players to attend their events, and limiting the amount they play in could result in the tournament losing a lot of money because people want to see big stars like Nadal and Federer. If there was a limit, they would only play in tournaments in which they recieve a great amount of points, such as Masters 1000 events and Grand Slams.
The top guys usually only play 5 or so tournaments a year outside of the 'Slams and Masters events, so they really don't need to be limited. For a top player like Fed or Nadal, the "minor" tournaments generally consist of the tournament in Qatar and or the UAE at the beginning of the year, a tournament in the player's home country, the Wimbledon tuneup in Germany or England, and the tournaments in China and Japan during the Asian swing at the end of the season.

