It doesn't have to bend to separate at the groove. For a fun experiment, try this. Grab a 1 x 1 x 24" piece of pine board with mostly straight grain. Tie a small rope around one end and tie the other end to anything strong. Pull on it like a fishing rod with a fish on the end, give it 15-20# of pressure, it wont break easily. Now, cut a small notch out of the end of that stick and tie the string to that portion and pull. It will separate almost instantly. That's why we use overlays or just cut grooves in the sides only. The way I see it, we strive throughout a build to make the most durable bow we can, why cut corners with string grooves? It would be a shame to have a great bow split because you cut one corner in the build.