Otherwise if they do the room only, for many people their only option then would be to live at the counter-service places (not that that's bad in itself), and would miss out on some of the better stuff.
We do the room only discount, and we eat mostly TS options. Still ends up being about $500 cheaper for us. Also, people need to remember that you are still paying for the tip. Be sure to figure that cost in when you do the math difference between free dining and room discount.
Aslo, we tend to share and app and dessert. This is not an option on the DDP. They really have made the DDP not as easy as it once was. Aslo, we tend to eat at at least 3 'signature' dining spots during our trip. Thus cost 2 sets of table service.
I will say this, if you are a family of 4 or more, and budget rarely allows you to enjoy TS meals, then yes, the free dining is a wonderful experience for those folks to take advantage of. It also would be more cost effective.
It's all a question of taking time to do the math. I save every reciept and place in a envelope at the end of the day. I include every little thing we spend money on during the trip.
When I get home I place it all in a spreadsheet. I break down how much the meal was based on alcohol, food, tip, all the snacks we ate. Everyway I looked at it the discount I got for room only was much higher than what I spent on food, plus alcohol, plus tip, plus all the Mickey bars and dole whips my husband has to enjoy each trip!
For me it's the math that makes it work. Each family has different needs and different ideas of how they want to spend the vacation.