The short answer to your question is that it all depends on your eating habits and your park plans.
Here's the longer answer: If your breakfast normally consists of cereal, bagels, fruit, yogurt, toast, muffins, etc. then you'll be fine with the concierge lounge. If you're an eggs, or big hot breakfast person, then you will need to factor that into your food budget. As for the rest of the day, it all depends. If you're going to be in the parks, then obviously you wouldn't get much use out of the concierge food offerings. If you're spending the day at the resort or coming back for breaks, then you could get all your cold drinks there and more or less appetizer type food. It all depends on which resort you're at as to what types of offerings are available throughout the day. If you're at the resort in the evenings, then you can enjoy a glass of wine or beer and from 8-10pm most of the lounges offer cordials, coffee and desserts. I believe that the GF and Contemporary are the only resorts that offer a second option of limited concierge service and that would preclude either the late night dessert offering and/or the alcohol.
My personal experience is that we do spend less money on food. We're fruit and cereal eaters for breakfast, so we try to stick to the same diet when we're on vacation. We probably spend a little more time relaxing at the resort than in the parks, so we supplement smaller purchased meals with the food offered in the concierge lounge. Unless we're away from the resort, we don't spend any money on drinks.
Hope that helps,
Tammi