And they wouldn't be making accurate guesses. DVC members staying on points are paying for the DDP. Lots of guests who got great hotel rates are paying for the DDP (works out to be cheaper.) People who booked too early or too late to get it free....etc.
But it doesn't matter if you got it free or if you are paying, the dining plan is the same. If you get poor service, talk to a manager. If you get great service (beyond your wildest expectations...) tip additionally.
Free dining is a resort promotion offered as an alternative to the deeply discounted resort rates that were previously offered during the same time period. The restaurants still get paid. I'll agree with the previous poster, if you get great service tip a couple of extra bucks, or at least take the time to let management know before you leave and if you get poor service immediately notify a manager. It only takes a minute or so to mention a great server to the hostess, maybe a minute more if they track down a manager so you can give a compliment to the right person.
Based on past experience any substandard service is either because of overcrowded restaurants or a server having a bad day. There are very few cases of guests getting bad service specifically due to dining.