Do you play SPG? If yes, you should know, how often you hit the target? Maybe 20%? So, there is no reason why to use AP ammo. Most players will use HE, because there is possibility to hit the target by splash. Someone will use ammo with stun, mostly in Clan wars or tournament, but for public probably not. So I suppose that 90% players will use HE only for DMG. Arty was nerfed a few years ago and this will make arty more and more useless.