There is a VERY good reason to add pre-ordered games to your list even when you haven’t physically received them yet:
The Marketplace will offer you relevant mods that you can buy while you are waiting for your game, but will do so only if that game is on your list.
However, here is the BEST reason to do it: It apparently REALLY annoys busy-body Pinsiders who can’t just keep their minds on their OWN lists and are bothered by what is on YOUR list. And that, my friends, is…priceless!