SodiumAzide 
12062012 03:43 PM 
Its a matter of comparative power. If you take 5 auto attacks in an exchange at lv 1 and you block 25 damage considrring an average 500 hp pool you have saved yourself 5% of your life in damge. If you consider the multiplicative power of armor with this, you can understand that this is a very potent talent. Especially for 3 points.
Edit: As AD becomes lower the power of a flat reduction is magnified. At 70 AD you are looking at a 7% reduction in damage which correlates to roughly 7 armor. Given that knowledge we can say that against a ranged character like say Jayce who most agree is a very irritating lane, who has a 46 base AD at lv 1 and 3 per level after. This talent gives you roughly the same mitigation as 11 or so armor. As you can see, you are already roughly sittting on roughly 3/4ths of a cloth armor for free, which is a 225 gold value. This compounds if you have the dorans sheild. A total of 11 points of reduction means that you have 23% reduced damage from jayce. Roughly worth 30 points of armor, which is a 600 total gold defense value. On top of the 5 armor you get from the shield, and if the subtraction is counted after armor, its value is exponentially higher.
