There are so many ways in which crates can make life better for people and for dogs. They keep dogs safer in cars, offer many dogs a quiet refuge, are a great help during house training, and play a role in preventing bad habits such as destructive chewing and counter surfing. Dogs who are comfortable in crates are more easily able to handle travel in hotels and staying overnight at the vet. The policy statement by the Association of Pet Dog Trainers (APDT) about crates  makes these points and also asserts that it is imperative to use crates thoughtfully and to introduce them to dogs correctly. I agree.
I love crates and use them, though I do have a few concerns about them. They must not be overused. I prefer that dogs not be confined in them for more than a few hours at a time on a regular basis over the long term. Many dogs choose to go into their crates and to stay there, especially at night, and I have no problem with that, even though the dog is in the crate for more than a few hours.
Dogs who find them upsetting should not be in them. A dog should enter the crate willingly, even happily. Dogs who panic in and around them should not be crated, and no dog should ever be forced into a crate. Many dogs can be taught to enjoy a crate even if they are currently hesitant around them. However, attempts to teach dogs to like them if they have a really strong negative reaction to crates is not always successful.
There are ways to introduce a dog of any age to a crate that make success more likely. Introductions should be gradual and involve a lot of good stuff such as treats and toys that the dog can associate with the crate. The APDT Guide to Crate Training  is a useful reference about many aspects of crates and crate training.
What do you think about crates?