"You get one shot at it, make it count" is my philosophy behind these events, and having it set more frequently with multiple rounds would just make it shallow. I have played ArmA2 very milsim way and after a while, i knew the missions by heart, knew the AI behaviour and started goofing off in different ways to make it exciting - like strapping 80 kg worth of ammo, m240 and bandages to my guy, then juggernaut it through a night woodland mission while getting repeatedly shot and patched up.
New years eve every month would wear out pretty quickly.
