In general there is little to say against open PvP, but here I have to agree with the others. Open PvP just requires someone to protect the citizens IC and the players OOC. Since the guards can't do that right now, perhaps the players themselves could act as a guard, until the system itself is fully implemented.
/guard
>Your request to become a guard has been granted.
Now one could stop any trouble-makers by just fighting them down, without having to inflict a /challenge. In the wilderness it'd be open PvP anyways, so if one fears an ambush or just doesn't feel comfortable traveling between cities, he could just group up with a bunch of friends or hire some professional body guards. But well, if it would be that easy to implement, we'd probably have open PvP at least in the wilderness by now.
PS: The punishment for those who get caught fighting in town would need to be much higher than a DP of 30 minutes, in my opinion anyway. So yup, it might be best to let the whole idea sleep until a prison map is available.