There are two different AC Values, and two different Attack Values. Avoidance AC and Mitigation AC (And their Attack Counterparts).
Both players and mobs have these values, with different formulas for getting the resulting number.
We can study (or see from dev posts) how these numbers are affected by various mechanics, such as agility, but in the end these values are compared against each other in the combat code.
Mitigation has no place in this discussion, it is completely unrelated to Agility (Avoidance / Chance to be hit).
Agility does affect your Avoidance AC. Anyone can see how the Avoidance AC number changes on their stat sheet by changing their Agility around.
How much higher Avoidance AC helps is really up for debate, it could be very minor, but it does help.
Consider this, the warrior Evasive discipline is a direct increase to Avoidance AC. Log in a warrior and watch this number change as you activate it. It increases it by a good amount (I think +50%). Parses do show that this makes a difference, but probably not as much as it should for such a large increase.
So, Agility does increase Avoidance AC, which does help avoid attacks, but it's arguably a negligible amount. Only Tanks should be getting the buff, and it's usually one of the first to go when you need buff slots.