At one time many of the Epcot restaruants (and DTD restaurants) did not particpate in this program. I believe it has to do with the management that actually run these restaurants. It's nice to see they've added most of them.
And Puffy2, you would be correct to say you would need to spend $250 to break even (it was more for us FL residents the past few years because the membership was more), but it's really not hard to do if you dine at a sitdown at least once a day with a family of 3 or 4 for just a few days! We've had the chance to meet up with friends and have dinner, so if you dine at a place like O'Hana's with a group of 10 you pretty much can break even from that one meal alone!
If you only travel to WDW once a year for a week or less and don't spend much on dining (which is tough not to do)..then the DDE card is not worth it for you. We do about 4 trips a year that range from 3-5 days each and we even stay at the S/D all the time (and they don't take the DDE card), but we still eat at a lot of the WDW restaruants so it's well worth it for us especially when we can meet up with friends.