Firstly it seems to take effect before level 21, if you join the Purist Guild (I did at lvl 16 and got the bonus against neutral) but hey I'm not complaining about that.
What I have noticed is that the enchantment appears to be applied similarly to job bonus ab, in that ab buffs overide it as it doesn't apply directly to you ab but rather it is a special effect. So if for exampl you have 27AB and with Holy Sword sword you have 30 AB. If you the cast prayer you have 30AB base and 30AB against Evil (at least that what the character sheet says havent tested with in game AB). This really makes Holy sword bonus for pallies a bit redundant because at lvl 40 the enchant bonus is +10 which is the same as a +7 weapon (albiet rare but they are in the game) +3 from prayer. I think we can all say we would prefer to have 13 ab in this case.
If this can't be fixed I suggest it is just appied as a 'normal' enchant bonus similar to Greater Enchant Weapon because I don't see it as a major problem to just give them the ability to hit good characters as well (I know it is against the RP point of the Pure Class bonuses
