Mirror Line and Mirror Point behave right now similar with other CAD packages: when you mirror a shape against a line or a point, a mirror instance shape is created, but after this you can delete the reference mirror shape.
The downside is that you cannot propagate after you create mirror, but on the other hand, in real life is less probable that you will move your line and mirror. Also, as the shape is mirrored by it's transformation, you can afterward modify mirror's values in a consistent way, as the new shape is not a "MirrorLine" shape, but a real clone of the rectangle, box, sphere, and so on.
If for example you will think later that you want to move the reference mirror line/point, you move them and create mirror again. But if you just wanted to create this reference just as helper, right now you can delete it nicely, without making your mirrors to be deleted also.