I love that metaphor in how it applies to testing because it's basically saying that choosing the right testing strategy is the same kind of choice you'd make when choosing a brush for painting a wall. Would you use a fine-point brush for the entire wall? Of course not. That would take too long and the end result would probably not look very even. Would you use a roller to paint everything, including around the mounted furnishings your great-great-grandmother brought over the ocean two hundred years ago? No way. There are different brushes for different use cases and the same thing applies to tests.