Less compact URLs, because they demand additional space in the human short term memory store, are thereby less easily remembered and reproduced. Unique domain names must be registered, and with this comes a sizeable fee, and maintenance of domains so that they actually refer to a document somewhere on the network requires that the author acquire a machine to be associated with the domain, or else contract for a service provider to alias the domain to an existing machine and path. For this reason, it is important that URLs, where they can not be compact, are in the very least clear. That is, if we are forced to tax the limited memory capacity of the human being, we should provide our URLs in a form which is conducive to easy storage and retrieval. The basic principle behind clarity goes something like this.
Humans are language users.
We work very well with words, in fact some have suggested that humans are, in fact, preconfigured for language. But humans aren't limited to storing single letters in their 7 memory units. Instead, they can organize letters into words, and other information more generally into chunks, thus crowding additional information into very small spaces. This is how a human might store the above sentence in memory. When the limited memory store of the human being is taken into consideration, one realizes that in order to make our URLs sufficiently clear for humans to remember and reproduce, we must take advantage of their ability to work with words and chunks of information as discrete items for storage.