Too much urine in a privy upsets not only the moisture content, but, more inportantly it upsets the carbon/nitrogen ratios within the pile. In a "moldering privy", this isn't extremely important in the long run, since the excess will drain out, although in both a Clivus system and a "true composting privy", this can be a disaster. The excess liquid collects within the container and, by displacing the oxygen, it shuts down the aerobic decomposition and sets up the pile for anaerobic decomposition, which is characterized by that all-too-familiar ammonia odor. The addition of duff not only covers the waste to help keep down odors, but also introduces soil microbes that aid in waste breakdown.
If, however, you're working with WORMS, too much urine may serve only to drive them out of the pile, although there is not enough clinical study regarding just how much urine they can tolerate.
Based on my own (not-too-scientific) observations, this country has raised a generation of people who really don't know how to pee in the woods! If there's no toilet nearby, many of these folks will try to "just hold it" until they return to civilization.