In defense of procrastination, when one waits a while before starting a task, there is the good chance that new ideas will enter the mind. As a case in point, one of the award-winning writers for Time magazine admits that he cleans his drawers and straightens his desk and shuffles other things while mulling over his topic to write about. His digression into office cleaning usually pays off with a creative angle for his story. Sometimes people do their best thinking in that "final hour." Of course, there is no margin for error once one ignites that adrenaline genius that only appears in the desperate hours.
Another reason for procrastination in some people is the fact that they are perfectionists. If they finish a task earlier, they are going to feel compelled to fix something or redo something. So, by waiting, they avoid the obsessive behavior.