I was thinking the same thing before I saw your post. We usually eat at a restaurant for Illuminations, either R&C (more often) or Spice Road Table because we like to sit and enjoy ourselves, eat and drink and it's way more relaxing and enjoyable for us. I priced the dessert party for us (4 adults) at $316 and I checked one of our R&C receipts from last month which was around $200 including tip, appetizer, entree and drinks. And 2 hours of relaxed seating on the patio, front-rail, prime viewing. I realize that it is not a guarantee to get an outside table, but we have every time we've been there. I would be just as content to eat inside and go onto the patio for the show. For this particular show/party, its not worth it for us.
Now, the Star Wars dessert party was definitely worth it for us! Everyone has different priorities and that one appealed to us and I would do it again.