There is definitely statistical relevance in different group sizes. 10 drives the variability down far enough that you actually get a representation of consistency, and you can apply statistics to. A cold bore challenge is less about a proven system, and more about the fundamentals being applied to a hunting type situation with a cold shooter. If you think it's a nonsensical waste of time, you either haven't honestly tried it, or you're an extremely fundamentally sound and slightly egotistical shooter.
I replied to this because I think it's beneficial to the thread. When referring to your most accurate rifle, if all you've ever shot is 3 shot groups try 10 sometime and see if it holds together.