Yes- this is a good way to start thinking about it. I did run the numbers before our family trip, and the DxDP worked well for our family. We did a whole bunch of character meals and we had 3 "adults" and one "kid". We shared meals a number of times (at non-buffet places), because it is a lot of food, which meant that we had enough credits for a few signatures as well as everything else. We are definitely a table service kind of family - mealtimes were for resting up in the AC and enjoying each other's company. We often didn't get the dessert that came with our meal (again, we just don't need dessert that often). We ate well, but didn't stuff ourselves, and chose what we liked rather than trying to maximize our spend, and we still came out ahead. Now, if we hadn't been character meal heavy, and if we'd been paying full adult price for everyone, it would not have made sense.
I just did a quick trip with my teen daughter, and the dining plan would have made zero sense, based on our plans and preferences.
I don't see it as much more convenient than buying gift cards ahead and charging everything to your room, but to each their own.