Why don't heals and buffs have 10% miss chance (more if you are debuffed)? What about battle disciplines?
Wouldn't it be more interesting to plan for contingencies if your heal doesn't work or you don't get more actions?
Does enemy evasion, level difference, parry, buffs etc lower hit chance further than you own hit chance?
Does this explain lower than 90% hit chance when you've been putting only Strength on your melee character?
If so, can your 20% hit chance go lower than 20%?