I would never get QSDP but I agree 100% with your post. We were a party of 2 adults and 1 child and ate at some pretty good 1TS credits last trip; Le Celier, Spoodles, Tutto Italia, Cape May, Crystal Palace, Play 'n Dine, and Big River Grille (not good

). Our average non-buffet dinner check was close to or over $100. You are also more likely on DDP to order extras like app's and drinks. I figure we spent at least $150 on tips for the week.
However that noted, those tips are earned by servers, and are based on what we "spent" even if the dinner item is covered under the DDP. Based on the cost of the plan (we would eat TS 1X per day anyway), we saved close to $300 because we ate like Kings.
I wish tip was included in the DDP, but realistically they couldn't do that any longer (the servers were getting basically nothing for bringing you a filet). So with that noted, I'm glad Disney came up with a way to keep costs down for guests and to give us a choice.