Just as Microsoft has file “shortcuts” which allow a file to appear to be in 2 or more folders, but all shortcuts tie you back to the one and only original file, so too there is value in having one board appear to be in two or more projects.
Use case: I am using projects for clients. I want to have a SWOT board for a client where team members can collaborate on the SWOT. But I also want one project that shows me all SWOT boards for all clients...vs having to go to each client project to see them.