When you start a game in the middle of a table and reach the edge, do you move all the tiles across to give more room on that edge of the table, or do you play that the edge of the table is the edge of the world?
Novelty answered this question well, but I think some people just consider this a house rule. It has been discussed before and one great idea somebody had was to play on top of a table cloth so the game board can easily be moved if need be. I know from experience that moving all the tiles can be extremely tedious and not a nice proposition, especially when you add multiple expansions. A table cloth makes it very simple and doesn't limit your board growth. Does anyone remember what thread that was?
Justification for the limitless board can be made when you consider computer versions of the game that have no boundries. Some examples are AsoBrain.com (Toulouse) and Xbox 360 (have not played Xbox version personally, but I don't think there are boundries). I hope this helps!