Here’s a feature I’d love to see in Rhino:
A property of each object describing object mirroring about one of the global axes, if any (none, X, Y, or Z). This would be a display-only mirroring; geometry manipulation occurs only on the side actually modeled. The mirrored side would be dimmed to show the differentiation. A global variable tells whether such mirroring where defined, is visible at all.
In ship design, there is a strong symmetry port & starboard. It’s usually only necessary to model one side. While we could select everything and mirror it, subsequent changes would have to be made twice. Seeing the other side without actually modeling it is the goal.