Students should NOT be graded for particitpation. Some kids are shy, introverted and just don't like talking in class. Participating is not the only way of seeing if your students are learning, some people just learn better by listening, why give them a bad grade if they know the material?
A final grade is not determined solely based off participation. It is a contributing factor and should be as verbal communication is a life tool that carries on into adulthood. How much of a grade is based off of class participation anyway? 10-25% seems fair. In that case, a student would not receive a failing/bad grade even if they didn't participate at all if they are well versed in other areas of the subject being graded. Educators should explain to students the value of critical thinking and how in their future careers they will need speaking & communication skills. Introverted individuals may need additional coaxing however it is not an excuse to not participate or feel like they are exempt because of their individual personality. I will go off on a slight tangent and ask how many relationships have failed because of poor verbal communication between the parties? We can not blame lack of communication on shyness however it can be said that an adult who lacks it may not have been graded on participation.