Sounds like you need to read Arcanaville's Guide to Defense.
The answer is no. Higher level and higher rank (until +6 and over) only gives accuracy bonuses. If you're already at 45% defense, more defense or tohit debuffs will not further lower the chance of you being hit.
However, if the enemy you are facing has a tohit buff power on it, then darkest night will take some of the edge off of that.
The damage debuff from DN will always help you out, but it will become less effective as you face tougher enemies due to enemy scaling.
Be well, people of CoH.
Ok let's say I'm shield defense and I'm soft capped. I've heard that darkest night isn't necessary. My question is, what if I'm fighting Lt's or bosses? Will DN lower their acc enough to basically soft cap me to them? I'm not good with the number crunching. Thank you.