I uh, kind of figured this one out.
See, it's a game where you roll dice to determine the strength with which you do an action, but if you roll a number that can be factored, you must factor it and apply the results to multiple actions. So if you try to attack someone and roll a 6, you don't actually attack them at 6 strength- you attack them at 2 or 3, and use the other number for some other action. Similarly, if they roll a 4 to dodge, they actually dodge at 2, and maybe use the second 2 to counterattack or something. I guess that could result in interesting counterattack chains.
So, why roll with prime dice then? No particular reason, but it does mean that if you roll the highest number possible, it's extremely significant, since you're potentially getting a 7 or 13 in something where a 1 or 3 is common.
If multiple actions could be used for flurried attacks on the same target, or if split defense rolls could be turned into counterattacks, you could have some very interesting (and computer-friendly) counter-counter-counterattack chains. Add cards or other fluctuating status conditions and the game could be truly intriguing.