I like the argument for a number of reasons:
- It beg's the question "Why do I reject those other 529 gods?" Generally there is no good answer for this other than being indoctrinated at a young age. You realize that you believe what you believe because of the circumstances in which you were born. This leads to the conclusion that the religion you follow has more to do with where you live than what is "correct". (Noting that there are plenty of exceptions to this).
- It drives home the point that humanity has been creating gods for thousands of years. Most of those gods have likely been created before your god had even first been mentioned. Any rational person would ask why did humanity move past those gods? You can never disprove the existence of a god, so what happened? Ideally, you realize that religion is much like any trend, just with longer than average turn around.
- If you follow the trend of gods, you realize that humanity has been generally moving away from polytheism towards monotheism. This makes you wonder why and conclude that it is tightly correlated to (and arguably directly caused by) humanity's understanding of the universe. As we understand more about everything, we don't need gods to fill in these gaps of knowledge. For instance, we understand why the Sun rises or lightning strikes. As humanity has progressed, one by one we killed the need for Roman and Greek gods. This leads you to wonder what role a monotheistic god fills and realize that the god is generally used to bring understanding to things we don't understand and reason to things we can't control. This is hardly a rational reason for a figment that "solves" all unsolved problems.
In short, I like the "(n-1) vs n gods" argument because when you think through everything implied by it, a rational being has no choice but to conclude that in all likelihood the religion they follow is not correct. Unfortunately, emotion plays a huge role in religion, making rational thought difficult. On a personal note, I was a theist for 20 years, backed by 12 years of formal study in my religion. Making the mental shift to realizing everything I'd believed was wrong was difficult to say the least, but years later my life is all the better for it.