My 2 bits (should be discounted to 1 bit)
Since you are a passholder, you definitely will want to consider the Tables In Wonderland perk (only need it for one passholder) It will pay for itself if you think you will eat table service more than 5 or so times- while *not* on the DDP - in the year. Of course if you plan on going to the Signature dinners (like CG, or Jiko's, etc) then it makes even more sense to do TiW since it's not cost effective to use up 2 dining credits for signature meals.
The number of restaurants offered on the dining plan is very extensive, there are only a handful not on the list - really just the Swan and Dolphin eateries. I think you can add dining right up until you arrive for your trip (someone correct me here) The main problem is if you end up in the free DDP windows during your trip. There's not much point to adding it on the fly during those times as you will be hard pressed to get any ADRs you want at the last minute. When free dining is in play the restaurants get very crowded. Not impossible, but difficult.
People do complain that using the DDP can be confusing as well, whereas paying in cash and getting the TiW discount taken off is as straightforward as it gets. Since you already have an annual pass, I would really think about TiW first - then you are in complete control of your dining decisions. Being on the plan (or having TiW) has nothing to do with your ability to get an ADR. But if you weren't hungry you wouldn't feel compelled to eat at a sit-down because you have the credits. You could order what you want from the menu without worrying about if it is on the plan (see: appetizers) or feeling like you need to order the strip steak again because it's the most expensive thing on the menu.