It was built for adding a third tier. It's 22,000 or so at the moment with the upper tier seats going in by the end of this season and a third tier can be added if needed taking it up to 40 odd thousand I think. It wasn't actually designed to be so big for MK Dons themselves to use, it was designed to meet criteria to host big games. If we had won the world cup bid for example it would have been used for that. It is also eligable for hosting a champions league final (with the upper tier) which comes down to a lot more than just the size of the stadium. There has to be x amount of CCTV cameras, access roads, TV facilities etc and their stadium was built to meet those standards.
You could kill 2 birds with one stone at full time and save on petrol by not making 2 journeys to an out off town retail park. Just a thought. For those that travel to the game a fair distance it can give them something to occupy their time whilst gates open. Just another thought. Must have advantages and disadvantages.