Concierge is not all about the food (although it is a nice benefit). The breakfast spread is very nice (cereal, fresh fruit, bagels, pastries, juices, coffee) and usually does substitute for brekafast for us. However, the offerings during the rest of the day are more snack-like and don't really susbtitute for a whole meal. If someone wants to try to make a meal out of the food, more power to them, but since the food offerings don't change every day, this could get a little boring after awhile. All in all, it would be hard to eat enough to make up for the extra cost (and I would not recommend that one try).
In addition to the food, the main reason we stay concierge is the pampering. It is the pre-arrival trip planning assistance, the separate check-in, the extra help with issues as they arise during a trip, the lounge availability for snacks and drinks, etc. All these things together add up and make concierge "worth it" for us. But, to each his own . . .