I absolutely agree that it is a very poor use of party time to be sitting in a restaurant. That said though, we love starting the festivities off with an early TS meal in the MK, around 4 or 4:30. That gets us deep into the park to avoid everyone pouring in while everyone else is pouring out. And it fills our tummies with a great meal so we can forget about food the rest of the night. We are usually just heading out around the time we want to line up early for one of the popular party meet & greets.
Additionally, party days in the MK tend to be fairly low crowd days. We love to spend the entire day in the park, and use that late afternoon/early evening ADR as our "rest" as we regroup for the evening.