I have just try to use new feature of Milestone in one of our projects.
My expectation was that it will stay visible even when we mark it as completed.
This is somehow useful when you want to look back on accomplishments which in some cases can have influence on future work as well.
If this sounds like a good idea, than perhaps it would be useful that achieved Milestones are not treated as regular completed tasks, but completed Milestone are visually presented on the active task project view (for example with checked marked icon), and within search are not treated as Incopleted.