By focusing on the planned outcomes rather than the action, you can focus on what needs to be completed 100 percent and can easily roll the outcome into the parent item, keeping the hierarchy consistent in your project. The outcomes cannot be overlapped within the Work Breakdown Structure. Laying out the planned outcomes in a WBS prior to the project and cross checking each element for duplication ensures you will have the necessary building blocks for a well thought out plan.
The level of detail in the designing of the WBS can be found in progressive elaboration. This removes the threat of focusing too broadly or to detailed. Progressive elaboration allows planned outcomes to be refined before work begins. This fails to work when the project reaches a point that planned outcomes can no longer be defined and the only items remaining are actions.
The WBS needs to be thoughtfully balanced. Elements need to be concisely grouped, and outcomes needed to be clearly defined in the project.
Want to learn more about work breakdown structures? Read Ann Gordon's three-part series, What is a Work Breakdown Structure.