You wouldn't believe the people that do consider tips but forget to consider them when determining if the DDP saves money, or not.
As you likely figured out, tips on a meal at Disney are considerable and can indeed burst your budget. I have had a few meals where our tip alone equaled what we'd spend on a night out at home.
When you dine on the dining plan so many want to maximize their credits by focusing on the most expensive items on the menu, even if that isn't what they most want to order. Well, the most expensive items on the menu, x4 (or however many in your party) can quickly ramp that tip amount up, higher than you'd tip if you'd just order what you truly want the most vs. the most expensive item. With the margin on DP saving money being so slim it's entirely possible for the minimal savings you see using the dining plan to be "eaten up" with added tip costs.