Do you mean the Tomorrowland Terrace Cafe CS, the Tomorrowland Terrace dinner buffet offered in the busy seasons, or the Tomorrowland Terrace fireworks dessert buffet? If you mean the dessert buffet, they assign seating. (You may not get into this event at all if you don't have it booked ahead of time).
If you were just saying that you didn't want to fight the crowds on Main Street for a seat (people stand on Main Street for that show, they don't sit; maybe that's where the confusion is) then I think the greatest draw for people who book the dessert party is the reserved seating for the fireworks show - the desserts come in second. I have never done it as I am not really concerned about a view for the fireworks for myself, and I don't think the desserts offered are worth the high cost of the event. If they were able to serve wine or cordials, I might try it, but no alcohol in MK.