A long time ago, I was a server. Standard at that time was 10% on buffets. Now it seems to be closer to 15%. I know that working both a regular restaurant and buffets, there wasn't a big difference in how much work I did. More plates to be cleaned off the table - and with more plates came more drinks to be refilled.
I tend to give 15 - 20% at most buffets now. It also depends on if I really have a server and if they are coming around. There are some buffets where someone says they are a server, but you may see them once or twice (no refills - or 1 refill, and not cleaning up plates as much, or a bus person may do it). I'm not sure those are really servers per se, but I always leave them something too. Just not as much as say, Ken or Donnell at Trails End. They work hard and are part of my dining experience.
I would have rather seen Disney raise the dining price to $50.00 including everything. Appetizer, tip, etc. Servers at nice restaurants won't do as well either, as some people will think it was all-inclusive...or may tip on the original $38, etc.