It does if you value your time in the parks and would eat in the parks no matter the cost.
But let's be a bit realistic about this, $75 (including tax for adults) gets you 1 Table Service Meal, 1 Counter Service Meal and 2 Snacks. Now many of the snack choices these days can be meals in an of themselves, so let's take a realistic look at what this can get you:
1 Table Service meal that includes: Drink, Entree and Dessert - Eating offsite, the Drink could run as much as $50 since alcohol is included, but let's use a reasonable $4 for a soda and yes, that is what most restaurants around here charge now. Entree for the same portion size and quality as most places, around $25, the dessert will run around $10. So for the meal you are looking at about $39 + Tax for a total of about $42 with tax.
Counter Service: Drink - $4, Entree - $10 - Total: $15 with tax
Snacks: These will vary, but let's take something like the Mac and Cheese things served at Animal Kingdom, around $8 each, for a total of $17 with tax
Grand Total Cost from Offsite for a similar amount and quality of food: $74
NOTE: All prices are based on food of similar quality and quantity to what you would find at Disney and are based on Southern California prices outside the theme parks. If you drink alcohol you would come out way ahead on the dining plan these days.
And don't forget the refillable mug that comes with the dining plan, really not seeing a huge price difference between offsite and in the parks on the dining plan unless you are choosing lower quality or to eat less.