Based on the original formula, which I am 99% sure is still true, it is important to note that if you DO NOT have any fighting skill (Actual weapon skill, Mage weapon and Magery, Wrestling, or Eval + Anatomy), then DCI doesn't help you at all.
Following the math of the original formula, if both attacker and defender have 120 fighting skill, then the base chance to be hit is 50%. However, if the defender has no fighting skill, then the base chance to be hit goes up to 350%. With 45% DCI, it’s still 241%. The devs stated somewhere that there is always a 2% chance to miss regardless of the math, but as you can see, 45% DCI makes no difference if you don't have a valid fighting skill on your character.
A couple additional points here as well:
In PvP, if your enemy is using a Mage weapon to get a valid fighting skill, then disarming that weapon will leave them defenseless versus melee/archery/throwing for the time the weapon remains disarmed, since they have no real skill. This means a 98% chance to hit that character with these attacks for a short time.
Also, if you plan on doing PvP, and you don’t have some form of valid fighting skill, you’re going to be at a serious disadvantage vs. melee and ranged attackers because they will hit you pretty much every time.
At one point I used a Mage weapon for PvP, but the chance of being disarmed is pretty good if your opponent is a melee class and know what they are doing, so I have switched to the Eval + Anatomy method.