Has it happened to us? Yes, but not since around 2003 or so.

On our last trip, March '09, we had ADR's but wanted to fit in a dinner at WCC on a spur of the moment decision. We got there around 6pm and were told that they had zero availability and weren't taking walk-ups. Going during
Free Dining I would definitely have ADRs. You have to plan them around where you want to be each day. If you know that you want to enjoy PM EMH at DHS, I'd try getting an ADR for a DHS restaurant for dinner. If you don't want to lose any "park time" avoid making ADRs at the resorts. With a little planning you can make ADRs and not let if feel like the ADRs are dictating your trip.