I guess it really comes down to, would people rather board earlier and debark earlier, or would they rather board later and debark later. I personally prefer the "earlier" option. Once debarkation day arrives, the vacation is over, and I'm ready to just head home, and have no desire to prolong staying on the ship.
Another consideration is that people staying in hotels pre-cruise have to check out by a certain time (like 10:00 or 11:00). If they can't board until 13:00 at the earliest, then they have to have somewhere to go, over lunchtime, while waiting to board.