OK, let's do a little math. We are 4 adults (well two kids ages 14 & 11, so they are considered adults).
DDP costs $46 per person, which would be $184 for one night of the plan.
If we ate at Boma with our Tables In Wonderland card:
$40x4= $160 (non specialty drinks are incl. in price)
$160x18% gratuity= $189
$189 less 20% TIW discount = $151.00 total
So, essentially then you are paying $33 for the snacks and counter service meals. I agree that you do save some money, probably somewhere in the $20 range.
Of course I can buzz off-site and save somewhere in the $80 range and since we are there 10 nights that would be around $800. That's a pretty healthy chunk of change.