It all comes down to preference. It is true that booking ahead gives you no advantage for
free dining. You still have to modify your reservation, and if the hotel is out of free dining slots, then you either change hotels or keep the one you want (and don't get free dining). Sometimes it's harder to rebook than book a new reservation, depending on how you have to modify. But if you want a certain hotel no matter what, and don't care if you aren't able to get it, then it certainly makes sense to book ahead. To address the other side of the coin, booking up rooms can and has caused resorts to fall into the excluded list at the last minute...on more than one occasion.
It's an eternal debate that really has no right or wrong answer, it's different for each person.
Oh, by the way...free dining isn't always the best deal. Always check the room discounts and do the math.