Remember that flash arrow has a -ToHit that is unresistable which means it basically counts as a defense for you. It doesn't matter what is trying to hit you whether it's an AV or a skull minion, it gives the full debuff.
Not sure how you are building your defender, but if you use clouded senses in flash arrow + Power Boost (Soul) you will have an effective ranged defense of 18.89 (15.14 + 3.75). If you just remove the proc and put in a -ToHit IO in there you will still have an effective ranged defense of 15.95.
If the 3% difference would get you to ranged cap then it doesn't really matter if you alert them since you can glue arrow them right after or PGA and put the minions to sleep. Emp and OSA can also make short work of any mobs you might alert with the proc.