Remember back in the days where Malphite and Ezreal were forgotten or treated as useless? I believe it's somewhere around mid-late Season 2 that people realized they're actually very powerful. (Malphite is actually pick or ban even till now, and Ezreal is king before the attack speed reduction removal with a win rate of 70%)
It's really weird when people finally noticed attack speed reduction is a ridiculous anti-ADC ability, and forcing an ADC out of a teamfight usually means a win especially during late game. It is here that I wondered: "Does attack speed reduction alone make them very powerful?". Looking through all the champions that have attack speed reduction, we have:
Gragas: As awkward as Anivia, you don't really notice that his Barrel Roll reduces like 40% Attack Speed should it hit, probably because you don't see him anywhere but top or mid. But seriously, 40% attack speed reduction basically means the enemy ADC's output is nearly halved.
Nasus: You don't really see this guy often especially when you go higher up in skill. Wither has the best attack speed reduction in the game and totally shuts down any sort of auto attack threat. People are more afraid of his 300 minion farmed Siphoning Strike though.
Nunu: And finally, this guy. Ice Blast does actually make him annoying and an autoattack character's threat. It's usually because of the movement speed slow more than the attack speed reduction though.
Honorable mention, Ezreal: Back in the days when Essence Flux is a nightmare for AD carries... Yeah, you know those days when you get hit by this and literally can't fight back for a while. Riot had to reduce the projectile size of this ability before totally removing the attack speed reduction from it.
Oh and don't forgetand too, it's the reason why tanks buy them, to counter enemy ADC output.
So after looking at Ezreal's attack speed reduction removed, I wondered, Malphite would probably be removed from permabanned status should the attack speed reduction be removed or reduced from his E. It is THE second most potent attack speed reduction in the game after all. What's more, it's AoE.