How often to units deviate from scripts?

[13lackGu4rd]

as for your ogiginal question, how often, generally if your scripting is good than never. however scripting can be faulty for a variaty of reasons: no targets in range, forgot to bring gems to commanders(for spells that require gems that is), already having the effect(such as your charge body example), the enemy being immune to your spell(like scripting cold attacks against an all undead army), etc.