Critical chain planning has a purpose for the 50%. It's not perfect-- but it's better than other ways, in my experience.

Wikipedia: "The justification for using the 50% estimates is that half of the tasks will finish early and half will finish late, so that the variance over the course of the project should be zero... Because task duration has been planned at the 50% probability duration, there is pressure on resources to complete critical chain tasks as quickly as possible, overcoming student's syndrome and Parkinson's Law."