I'm endlessly amused by the comments of the form "I had late dining with my very young kids and they were fine, so I don't see what the big deal is".
I've always thought that people who assume that their children's "good" behaviour is a direct result of their super-awesome parenting skills simply haven't had enough children. They haven't encountered that one who makes you question everything you thought you knew about parenting. Trust me, if you had one who couldn't handle late dining, you'd understand why it can be a big deal.
When my kids were young, I wouldn't have booked a cruise if I couldn't secure main dining at the time I made my reservation. Or if I did, it would have been with full knowledge that we would not be having dinner in the main dining rooms. My daughter might have been able to handle it, but my son would have been a wreck. Even main dining was a stretch for him. He was five on our first cruise, and even then I had to leave dinner before dessert every night to haul him off to bed.
That's not to say I think there's a debacle, or a problem to be fixed. Just that it wouldn't have been a great vacation choice for my family at that time, so if late dining was our only choice, we would have chosen a different vacation.