However, architects don't use the plans, sections and other two-dimensional skethes just because they can visualize the things in their minds. The perspective renderings are just slow to make, and they can't be used for providing a complete view of the design, as they are just views from fixed positions.

However, architects and designers have been making three dimensional sketches for ages as well. Using cardboard, scrap wood, clay and whatever materials available, sketching is also possible in three dimensions. These scale models are important for evaluating the project in full 3d. Model building is however much slower than sketching with pen and paper and thus doesn't support the same thinking mechanisms. It also requires all kinds of tools and materials and moreover, communication with these physical objects is more difficult since they can't be copied or faxed to the client. Because of all this, the first cycles in the design process usually rely on pen and paper, and 3d comes later when the initial ideas have become more concrete.