It's basically true - at this time you cannot purchase a
free dining package that encompasses Christmas. You can, however, purchase a regular package reservation at CSR that includes a dining plan or you can purchase it with a DVC reservation. The inavailability of free dining doesn't have any affect on the general availability of the dining plan itself - the plans are always available as a package add-on.
We have a lot of dining plan diehards on the DIS Restaurant Boards - they love the dining plans and won't go without them anymore. I'm the opposite. I don't like the plans at all. I don't like Disney telling me where I can eat and that I won't be able to go to Bluezoo or eat appetizers at La Cava del Tequila because I can't fit that in along with places that take the dining plan. Nor do I like having to think about exactly where I am going so I can make sure I use up the credits which equal food I've already paid for. Every credit you leave behind is money in Mickey's pocket rather than your pocket (or your stomach) - so you might have to consider that if a party member becomes sick or just doesn't want to eat at some point.
But for certain types of guests it works. For those who do not want to think about what they are paying for food, and those who like dessert with every meal, it's the greatest. In the OP's case, with a teenage boy and a preteen girl (both adults to Disney) it'll probably revolve around "how much do we eat." If the family likes to take time out for restaurants, typically eats a load of food, enjoys buffets or high-end entrees and always or almost always orders dessert, it could be well worth it to buy the basic dining plan. If the kids (or the parents) don't eat much or prefer the type of food that's on the children's menus, or like to order appetizers in lieu of entrees, it might not work out so well.
If you have a car, you might also be thinking about eating in offsite restaurants, which would mean you have access to a boost in food quality plus a decrease in prices.
You should also consider that if you are thinking about Christmas/New Years 2009, it's going to be necessary to try and make restaurant reservations ASAP, and even then you might have no access to your first choices or even your second choices.