I haven't been to WDW yet, but did sign up for the DDP b/c for the cost of it for my children alone, it was a no-brainer, they'll eat way more than $9.99 a day, and for my DH and I, it's probably about right. My experience is from cruising. On our honeymoon, we had the same table and server at dinner every night and we tipped at the end of the cruise. 10 yrs later he was without doubt the best server we've ever encountered. Fast forward a few years and they've changed the policy: a set price per person/per day to be pooled and divided between all the workers, cabin stewards, servers, Maitre D, etc. It was a very noticeable difference in the service in all of the restaurants every time we've been since they changed the policy. Unfortunately, many people don't think they should have to work for their pay, they just show up, clock in and collect a check. It seems that's the way the waitstaff has become on the cruises. It is definitely more convenient to have it all included, but at the same time the when it's not included it's a motivating factor to the server to work harder, they could actually earn more than 18% rather than just coasting to whatever they will get.
I don't mind tipping for a job well done, been there done that. I will have a hard time tipping 18 to20% for dinner at a buffet. But since we haven't been yet, I'm only anticipating what it will be like.