Have you ever seen the "Entertainment Book" or what ever it is called. If you take advantage of the savings for their dining you agree to allow the gratuity to be placed on the check. I used to work for my DH cousin and he accepted the coupon, but I did not add the tip on. Now our restaurant was moderately priced. We got a lot of people who only ordered the entrees, no appetizer, no cocktail, no beverage and no dessert. They paid for one meal, got one meal free and a good amount of those people did not tip on the amount the bill would have been.
A lot of people were bargain shopping in restaurants, getting decent meals and keeping the price down. They were not regulars so they had no issues about us "knowing" them,and they were clear that they were keeping their bill down, I was just the means to the food, not the person who was enhancing their dining experience.
I am not saying that everyone does this or that all of my customers who used the coupon did this, but there were enough that I was not always happy when my section filled with these customers, and my perception to this day is that my tips were negatively impacted.
FF to me going to an area restaurant with my Sis IL and taking out my coupon. My Dh and I frequent this place a lot, he just won't go with the coupon. The server just about ignored us through out the entire meal, it was kind of busy. He did not even bother to ask if we wanted dessert, coffee or another glass of wine. I think he was used to getting told "No thanks" from the majority of coupon diners. He did beat feet when we got up, cleared out our things and went down the stairs. I complained about the level of service to the manager, but that did not improve the quality of the experience, just brought attention to the management. The manager said that this man was usually very attentive, I believed him as the server was giving a lot of service to the people around us.
If there a too many "bargain" shoppers buying the plan, stretching it out and taking tables without compensating the servers appropriately the level of service will go down. Then customers will complain, and rightly so. Then the domino effect will occur. Disney used to pay the gratuity. This did not come out of our pocket and yet there were a lot of people who resented an automatic tip. Now the plan is ever so slightly decreased in price, the value is decreased and the gratuity is not included. If there are too many people not compensating servers there will be complaints....not from customer....from servers who do not want to take these customers. I foresee the tip being adding on the final bill. Oh sure, Disney will let this go a year, but I can imagine that the purchaser of the DDP will be agreeing to the automatic gratuity down the line so in fact while "we" got what we wished for, we may end up paying for it after all. There is no "free" lunch. Ever.