2D diffusors just disperse sound horizontally (or vertically if you hang it that way.) A set of books does some diffusion and also absorbs some. The idea of an exact set of heights is that it spreads things more evenly throughout the particular plane than something purely random will.
3D diffusors (the grids) are designed to disperse in both horizontal and vertical axis - more like a spherical dispersion pattern.
Diffusors also are somewhat absorbtive due to some minor cancellations that are generated.
As usual, something is better than nothing. However, with all the plans out there on how to do it, it really isn't that bad other than a lot of time for construction - especially for the 3D ones. The material costs aren't bad. The biggest thing people have issues with are the looks and the depth they take up depending on how low you want them to work.