The idea here is that an opponent will want to prevent damage but not have each blocker shrink. Sadly, flanking is not a viable core theme by itself. Paired with other things (first strike, for instance) it can be effective, but it remains a better adjunct, and its value is fading quickly in the face of much cheaper creatures. The other problem is that flanking doesn't come cheap. The 4/9/20/7/2 curve is too expensive at an average of 2.86.