Sounds a little weird, and you might wonder why I would need to copy a plan from one project to another.
Well, sometimes I like to have a Detail Project or a Learning Project. Inside this project will be snips of details, that I can reference later without having to remember which project had this type of detail.
For example, there is this detail for bollards that look pretty cool to look at later

I crop it as a new page, for that plan. But if I want it later, I would have to print the plan, then upload it to the other project. Just a bit more work, not a biggy. It would be faster if it was a right click that plan page and copy to existing project or new project.
Another example, would be if a plan really sucks and lacked detail on an item. You could go to the detail project and move the detail into that plan. and use it as a example submittal.
Also, if you have city std plan, you can move it into that plan without having to add page.
Extra Feature:
Have a right side template folder that has details (Maybe called Detail Temp) that can hold these details without having to make a separate project. Hovering over the item will pop up a small screen with a preview of the detail.
Or if I need to explain what core drilling corners to avoid overcuts, without having to google a picture. I could just go into a detail project and send them a link to that page (FYI, couldn't even find a picture on google, but did come across one while bidding)

Also, while I am at it, how about copy plan page or duplicate plan page