Booleans are fine in some situations and actually work brilliantly in CAD packages or other types of "solid" modeling software that use a Delaunay triangulation method for the mesh, Voxel modeling, or Nurbs based modeling. In those types of conditions, booleans work very well and could be retopologized into quad meshes. However, when working in a polygon software package like Blender, where most of the modeling features and techniques rely on quad based surfaces for the mesh topology, boolean operations can all but ruin the quality of the surface, and often require lots of clean-up, because they tend to leave behind isolated vertices, Ngons, and other issues which could give you trouble down the pipeline. Especially if you plan on transferring the object into other applications. If you are familiar with boolean "clean-up" then it's not really a big deal, but most of the time there's a much better way to create the same shapes.

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