Interesting idea, and nicely coined. As long as teams or individuals set modular and attainable (hard, but attainable) goals, one-dayers put a constraint on participants, and stimulates the subconscious to strive for better focus and communication. Reaching the predefined goal in a day is its own reward. I can imagine myself working with more zeal for that satisfaction at the end of the day. One-dayers can also be used for scoping/investigative work: they are short enough in duration to not be crippling in the larger scheme of things.