In the following clip, we can see that the "head lock" was applied 8 times with 4 succeesses (0.30, 1.17, 1.56, 2.44) and 4 failures (0.55, 1.44, 2.27, 3.44). the successful rate was 50%. This should be a valid proof that it's a very useful skill in the Sanda/Sanshou tournaments.
If you compete in Sanda/Sanshou, how important in your mind that "head lock" should be one of your effective tools?
http://www.youtube.com/watch?v=dbzghEV0rIM