I train MS project to commercial companies and you are right in that if anything has a duration it is a task and some eefort is being expended to perform it. However there maybe the case on the contract where something special is happening and work has to stop to allow this and then restart again. You have to include the down time in the plan, so because it is a major event you use a milestone.

The example I use is that I am handing over part of a construction site and have a celebrity unveiling a plaque. I do not want to have machines and tools going off so I suspend work during this handover and unveiling and then restart after it is finished. Obviously this is a key point reached and is major so it needs to be a milestone, but it also lasts for a period of time so we set a duration. But remember no effort or work is being performed.

Hope this helps

Tamsons