If one plans to dine at a Signature every night, then your 3 daily credits (if you are on the DxDP) would be used up by that dinner, plus one other meal during the day. Which means that one meal per day (probably breakfast) will be OOP. If you don't buy the plan, you would also be paying for breakfast OOP. So breakfast is a "wash". So the question becomes, can you do a CS lunch and a Signature dinner for less than or equal to $80 per person per day before tax and tip. Shouldn't be that hard. ($12 lunch+$12 dinner appetizer+$40 entree (which is high for an average)+$10 dessert=$74). At a minimum, you will certainly be very close. And not having the plan opens you up to the very best restaurants on property. So even if you have to pay just a hair more, you will eat better.
As for "paying in advance"...well....that to me has always been like setting your clock ahead 10 minutes so you won't be late. "Oops. Fooled myself again!!" If it is really that important, pay yourself by making a deposit in the bank, and then put everything on a credit card. When the bill comes, use the money that you paid yourself. You get credit card points plus a bit of interest. Or buy a Disney Gift Card in the exact amount that the DxDP would have cost everyone in your family, and pay all your meals with that. You get the same carefree feeling of having pre-paid.
Really, it all comes down to how important it is to you to dine at the better restaurants. If it isn't that important, then none of this matters. But if splurging on the best is really important, I can't see going to WDW and skipping the best restaurants just because I had to be tethered to a plan.
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