I can see how this would be very good.
I can imagine going to and from different genres in the lego universe such as City, Space, Castle, maybe even Star Wars.
You could build anything you wanted in the game, just like with real Legos. If you didn't have all the right pieces, you could buy the pieces you need using in-game money (in Lego Star Wars I think the money is in the form of Lego studs). Or you could trade pieces with friends.