KYS, I'm really sorry to disagree, with you especially, you give advice and support from the heart, are kind, loving and one of the best kind of dog people there is. But....!

To compare a cage to a den is very confusing to me. In the wild a puppy will have litter mates and a babysitter. By the time a babysitter is needed the pup will be out of the den and spend most of his awake time playing and exploring, only retreating to the den when danger threatens. On fine days most sleep will also be outside of the den. Before a babysitter is needed the pup will have the almost constant attention of his mother and the constant companionship of his litter mates. The den always has a roof and solid walls. This is (my original opinion, again!) behaviour "specialists" looking at owning a dog in the easiest way possible for owners with little real thought or consideration.
In an emergency such as a fire in the home when you are not there how is the dog being shut in a cage going to help it or keep it safe? Dogs in this situation are trapped, not helped. In an earthquake things for the dog, if anything, get even worse! There is so much research now that few people can argue the fact that animals, including dogs, know when an earthquake is imminent. To put a dog in a cage, let it fret for three hours knowing that it is trapped and what is coming rather than it fretting and letting it choose it's own best place to go for survival doesn't sound like a great idea to me.
At the vets your average dog that is crate trained does not settle down to sleep and feel safe because it is used to being in a cage. It is still anxious and worried about the fact that almost every time it goes to the vet it means something unpleasant is going to happen it.
Travelling in a car should be second nature to any dog that has a car in the family. Putting a cage in the back of the car to protect the dog has nothing to do with the crate in the house. If introduced even late in life to a dog that enjoys car travel the cage has no adverse effects at all. If you have a dog that you think fears the cage in the car, it doesn't. It is the car that is the problem.
Air travel is not going to be less stressful for a dog used to a crate. The only way to get a dog used to this kind of travel is to do it a lot from when the dog is young.
Putting the crate in the centre of household activity is another way to disprove the "den" theory. Dens are the place to go when you are in danger or need some peace. If your den is constantly surrounded by people rushing around, kids playing and other animals going past you move dens!
Comparing a cage to a den is very popular and some trainers would have you believe it is the only way to treat a dog like a dog should be treated. I don't think so. I think this is what a lot of people want to hear as it ties in their desire to own a puupy with the lifestyle they live. It is a quick fix, some dogs will cope. My question is should they have to?