In the event of a cable cut, a ring can wrap around itself. Any more-complex topology requires routing. Imagine a network with nodes in Boston, Miami and San Diego. The answer is routing. And it doesn't exist at Layer 1, at least not yet.
Some optical advocates say routing decisions belong at the edges of the optical network.
That's true, as long as all-optical networks remain small and simple. If switch makers are serious about increasing sales, they'll need more intelligence in their boxes. The other major roadblock to all-optical networks is figuring out a way to buffer light. No optical device can slow or store light, the way electrical devices buffer packets. This makes all-optical devices impractical anywhere congestion exists.