It's also important to separate the plan and communicated message. The plan should be without buffers but the message communicated should have buffers depending on the punishment of divergence from the plan. If the punishment is huge the buffer should also be huge, but if the punishment is close to none, the buffer could be the real estimation.

The reason for separating internal plans and status updates from what is externally communicated is because plans, which include buffers, seem to result in low energy in the team. There is even a law articulated for this. It's called Parkinson's law and goes like this: "work expands so as to fill the time available for its completion". That means that if we include buffers in our plan, the buffers will be used.

In cases where the scope may vary, you can decide how much to communicate. Let's say the marketing department needs to know what will be delivered so they can start producing a future campaign. Then it's good risk handling...