Do DCC clerics have the same issue? Is divine magic unpredictable? Psionics?
Given that DCC seems to have incorporated this from the ground up, how would you adapt unpredictability to arcane magic in D&D (3E/3.5) ?
Cleric spells also variable effects, based on the spell check roll plus mods. For example, here are the lowest and highest success for "Divine Symbol":
lowest: For a duration of 1d6 rounds, the cleric can attack with his holy symbol as if it were a magical weapon of +1 enchantment. It deals 1d8+1 damage (modified by Str, as usual), with an additional +2 damage bonus against unholy creatures.
highest: For a duration of 1d6+CL days, the cleric can attack with his holy symbol as if it were a magical weapon of +4 enchantment. It deals 1d20+5 damage (modified by Str, as usual), with an additional +4 damage bonus against unholy creatures. In addition, as long as the cleric is visible to his allies and followers, they receive a +2 bonus to saving throws and morale checks. Finally, the cleric also receives a +6 bonus to all spell checks to turn unholy while using his holy symbol when under the influence of this spell.
Deity Disapproval can be atoned for. Corruption has no mechanic for reversing, rather it is up to the player and GM to find in-game means (e.g., pacts with demons, difficult and elaborate quests to collect esoteric items to develop an elixir that *may* reverse *some* corruption, perform difficult and elaborate quests to obtain divine favor, etc.)
Also, cleric spells are not mercurial.
As for incorporating it into D&D (3/3.5), I have no idea. Instead, I would port my game to DCC -- YMMV.