You say : remove AS cap, but complain about getting your AS reduce by 20% ???

In the math equation of your total AS, 20% isnt that much, because it is 20% OF YOUR BASE ATTACK SPEED. So if you got item/mastery that gives you 120% AS, and your base AS is 0,5 (round number) your attack speed is : 1,1 attack per second. And of some one had a randuin, your attack speed would be : 1,0 attack per second.

Whatever item/runes/mastery, if your base attack speed is 0,5, attacking a unit with a randuin will reduce your attack speed by 0,1. Significant if your not adc (and got lower than 0,9-0,8 AS) but if your going full dps mode nocturne, and finish your build with a lot of attack speed, it wont matter as much.

Yes it counter a bit, but doesnt counter much.