Basically, by removing the dessert and the tip, Disney decreased what we get on the plan without changing the cost. It went from being a great deal to being an ok deal.
Obviously, you meant by removing the appetizer and the tip. The dessert is still included (entrée, dessert & fountain drink). But otherwise, you are absolutely correct. This move was a way to increase the profit for Disney (not the restaurants) on the DDP without changing the price for guests.
I realize that by not having to include the appetizer in the meal plan, that is better for the restaurants, but I would believe that Disney is just not giving the money they used to get for that. I am positive that the servers wish that they were still included as this would mean bigger totals - higher tips.
keep in mind that since fuel shot up to nearly $4 nationally, food followed right along with it. There is no reason to believe that it isn't more expensive for disney/the restaurants too. While they could have passed those costs along to people who are getting the DDP, this was a way to keep it at a specific price point (which is something I am sure that they are doing).
It is funny, because the price of park tix goes up basically every year - and everyone basically expects it. Food is different. I am positive that someone did a survey to determine that the were ceilings for the meal plans under $30 for the quick serve, under $40 for the Standard plan and under $70 for the DLX. Again, I am pretty sure that if it was $31, $41 and $71 respectively you would lose a huge number of people signing up for the plans.
So that is my 2¢ as to the reason for the no appetizer and tips in the DDP. I am sure that there are other factors, like the yearly negotiations with the non-disney owned restaurants. But in the end this is a very complex process for disney because it involves so many people and organizations.