How would it be pointless? For those wanting to fix the price of their trip, they order the non-supplemented items. For those wanting better quality foods, they order the supplemented items, paying a small "fee". Finally for those not on the DDP, nothing changes and the variety and quality is still there.
I understand what incurring fees implies. I am not confused on that part, because incurring fees may make sense in other tourist destinations. But more in detail of my point is the following:
Firstly, because the DDP was originally made as a way for pre-paying your meals. Once they make people pay supplemental fees for more expensive items, people will notice and not return to the DDP. Trust me, once they start that, the DDP is pretty much over for them. And regardless of what people may think, Disney reels in a lot of their American tourists with the DDP.
Finally, what would be the point of getting the all-inclusive pre-paying "cost-saving" dining plan if they incur fees? Their whole marketing approach would need to be re-worked, re-tooled, and pretty much re-hauled. People would not be too keen on the assentives anymore (not like they are 100% okay with it now), thus more people eating offsite.
Now, with Disney boasting about attendance at their parks, they don't release this big detail too much: their in-park revenue (such as souvenirs, food, and tours) is slipping a lot. Why would they want people to start eating offsite, or their less than expensive CS? Even though people "save money" on the DDP, disney still gets you with your pre-paying on the DDP.
Anyway, long story short, I just do not see them incurring a fee, unless they plan on doing away with the DDP completely, which in spite of what I may sometimes read on these boards, I just cannot see them doing that right now.