Yorkers are game changing balls.
Any bowler. On any pitch. At any time. You turn an innings around with the use of a good yorker or two. Ian Pont thinks they are so important that mastering one gives you a blank cheque for the IPL. But it's not just at the end of a Twenty20 game where they are destructive.
A yorker is a difficult ball to negotiate, even for well set batsman. You can bring it any time you want to upset the guy at the other end. You can use it to break a big partnership in longer games just as effectively as you can keep runs down at the death.
So if they are so useful, why don't we see them used more often?