We always do the DxDDP. DH won't even discuss the possibility of taking a trip without it, lol! I don't actually know if it saves us money, in fact I don't even like to think about the individual prices for anything (tickets, room, DDP, etc) I just have a whole budget for the trip and make sure we stay inside it.
So it's not so much about the big picture money for us, but more the guilt at individual meals of "Oh, I guess I should get the veggie lasagna instead of the huge steak I really want, b/c it's half the price," or "Well, I guess that really yummy looking appetizer isn't really worth $12, maybe we'll pass". And then DH will be sad and sulk about it for the rest of the meal. Or beat himself up the whole time if he gives in and gets it anyway. With the DDP, it's all the same, so you might as well get what you want. It's not just about having it prepaid (like on a gift card), it really all feels "free" at that moment. It's the kind of stress that DH loves to be able to not have on vacation.
If we didn't do the DDP, and paid out of pocket, I would have a hard time making as many TS ADRs for the same reason. Even if the $$ came out the same in the end, I would start feeling guilty and trim a few here and there. Which would no doubt make me sad later. Being on the DxDDP especially, we need all those ADRs to feel like we are using it well, so I can plan as many TS meals as I want!
Even if we were calculating how much we would spend and putting on a gc ahead of time, we would still of course know that if we could manage to spend less, we'd have that money for other things... and it would create this pressure to try to be frugal. I try to be frugal in daily life, but it's one of the things I want a break from on vacation.
So we get to eat what and where we want, and not feel guilty while doing it. Because it's all been paid for months in advance, and is now use-it-or-lose-it. It's a win-win.