A couple of things I considered.
1) You "can" get parking reimbursed with the DDE. Since you have seasonal passes, that could save you $10 per visit. In order to do this, you have to use your DDE card within 3 hours of the time you enter the park, and take your parking receipt and meal receipt to guest services. They will reimburse you. I've never done this, but it is legal.
2) You need to consider the savings as an apples to apples. As an AP holder, you can get a 10% discount at some of the places that take DDE.
3) DDE gives you some counter service discounts too, so take them into account when calculating your savings. All the value resort food courts do this, as well as the CS restaurants in AK.
I have to say that I made back the $60 savings within the first trip (6 days over spring break), even adjusting for the 10% AP savings I would have gotten. My family don't drink, and are fairly light eaters and probably only eat table service 30-40% of the time we visit. And we've probably saved $250 within the year.
In my opinion, unless you almost never eat at table service restaurants, if you're going to be at WDW enough to justify APs, then the DDE would be worth it for you.