I think that it would be great if we could rotate buildings, making it a lot easier to fit some buildings in, and making city planning more flexible. I don't see why it is so much easier. Many buildings are square:
decorations
2x2 Houses, large decorations
3x3 many cultural buildings
4x4 stone, granite, limestone, etc, pool.
So rotating these would be only for design. But I must say that have not had much problem fitting rectangle buildings onto the grid, without being able to rotate them. And there was a time I was not aware I could even move them.
I think the thing to consider is that people will always want a greater and greater amount of flexibility from the interface, at all levels. Hardly anyone asks to remove options from the interface. So the idea is only a natural one, in my opinion.
But I can see why graphically, whereas many of the buildings have moving people in the foreground- it would sort of be silly to move them backwards when the people are working in the front. For example, the brickworks (in my second monitor) has a nice waterwheel with water moving, a hammer machine, and a person working. All three would be lost if the building were flipped.
I just thought I would present both sides of the argument in a balanced way.