There's an important point that is always left unsaid in posts like these, and it's why I disagree with using such absolute language. Yes, a lot can happen. Every day, there are horror stories of diverted flights, cancellations and airport meltdowns that causes guests to miss most or all of the scheduled beginning of their trip. In the majority of cases, however, everything goes relatively smoothly.
That's why, like so many other things in life, this is a risk/reward proposition. The no-show fee for a dining reservation at Disney is $10 per person. You have to decide whether you're willing to stake that money, knowing it's possible you lose it. Your decision will depend on plenty of factors, including how many people are in your party. I might be willing to gamble $20 on a reservation for just my wife and me. I might not feel the same way with three kids in toe, upping that amount to $50.
I just think it's important that we lay out what we're talking about here. For some people, a coveted Be Our Guest or 'Ohana reservation on arrival day might be worth the risk. For others, not so much. There's no right or wrong answer here.