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But by that logic, all spell penetration would increase the damage he deals, i.e. sorcerer's boots, haunting guise, and void staff. I have never heard of those types of items increasing his despair damage.
Due to Amumu's passive and the way Magic Resistance Penetration and Magic Resistance Reduction interact, those items are much less useful to Amumu then they might be otherwise.
Two important differences between MR Penetration and Reduction -
Reduction can reduce the enemy's resistance below 0. Penetration cannot
Reduction & Penetration are applied in the following order.
* Guessing, because I don't believe there is any % reduction in game. It would be almost the same as % penetration anyway.
Note that Armor Reduction/Penetration works the same way.
Amumu's Passive is 'Cursed Touch' - his auto-attacks apply a MR Reduction debuff on the target (20/30/40, based on the mummy's level). Many Champions start with 30 Magic Resistance, and don't gain any per level. Thus, most non-tanks will be brought below 0 Magic Resistance if Amumu auto-attacks them.
Since Penetration can't drop a target's MR below 0, it's fairly worthless for the mummy against anything but another tank... which he has Despair for anyway. This is also why the Abyssal Scepter is so useful - dropping people even further below 0 MR makes magic damage very painful. It'll also help counteract the last remaining MR targets might have gotten incidentally (Chalice of Harmony & Mercury Treads are common items with a touch of MR on them).
It also helps that the Abyssal Scepter has a chunk of MR on it as well - since Amumu is usually built to tank, it has stats he was looking to pickup anyway (AP, MR, the -MR Aura is a nice bonus for the combine cost) . While Haunting Guise has Health & AP on it, the Penetration and Spell Vamp arn't all that useful.