I'm not sure if they used to hold tables for walkups and stopped when they started the Dining Plan, but it's really gotten tough lately. Last April, during a really-not-too-busy time, my Mom and I hadn't made ADRs, and boy were we sorry. We tried to find someplace to eat at or near the GF around 5PM one night, and were told by the concierge that not ONE table service place at the GF, CR, or Poly was taking walkups for the rest of the night.
At this point, I'd recommend ADRs for any TS meal that you want.