The ability to show Levels and Grid Lines in 3d isometric views would be extremely useful when creating isometric plumbing risers.
Currently, you have to manually draw detail lines and use text annotations to indicate levels/grids.
The way I would enable the fucntionality is make it so datums will only appear when you LOCK the views 3d orientation so you can begin tagging.
I'm not sure if this has been posted before, but I figured I'd post it anyway (I did try searching).
I second this.... The only accurate way to do this is to create a cross-hair in Autocad, insert it into the model on the floor in plan view, copy it everywhere pipe goes thru floor, and turn them off in every view but the 3d. Looooong workaround.