"Crate training" vs dog-in-a-box

For tens of thousands of years, dogs lived with people and moved freely in most households or out of doors. In the past ten years, something called "crate training" has become ubiquitous. Different people use this term to mean different things. For some people, this merely means "training a dog to accept short stays in a crate when necessary". For some people, it means "using short stays in a crate during puppyhood to teach the puppy to hold its bladder for a while".  Some people use a crate, open or closed, only as a night-time sleeping place for their dogs, and some dogs enjoy this.  A crate can be useful to prevent destructive and potentially life-threatening ingestion of household objects during brief human absences.

For an unfortunately significant subset, however, "crate training" is used to mean "Keeping your dog in a small box 24/7 throughout its life".

"Training", properly understood, implies a relationship. Keeping a dog or a child in a box is a way of avoiding interaction. It is not training, but the opposite of training. Training requires time and attention and use of your brain. Keeping a dog or a child in a box is relatively effortless and quite brainless. It would seem to be a great option for people who do not actually like dogs or children, but for some reason feel they must have them.

The cost to the dog or child is failure to develop normal cognitive, physical and social functions. The harm is profound for dogs, who are hard-wired to be social beings, and who need much more exercise than do people to maintain physical health and psychological well-being.

For a good discussion of useful vs harmful use of crates, see The Guilt-Free Dog Owner's Guide by Delmar and Lawson, p59-62.

Often linked to excessive crating is the idea that it is acceptable to force a dog to wait as long as 12 hours at a stretch to empty its bladder. Bizarrely, people rationalize this with the claim that the dog CAN do it, and therefore should be expected to do it. 

A person could hold his or her bladder for 12 hours, too, if the cost of failing to do so was soiling themselves and then being punished by someone much larger than them.  Dogs  have no less need for bladder relief than we do.  A dog who is routinely denied adequate relief suffers continually, and will be subject to the same problems a human would, including increased rate of urinary tract infections.
Owning a dog is, quite simply, inconvenient in many ways.  This inconvenience cannot be wished away by pretending that dogs are inanimate objects lacking biological and social needs.