Allied Opportunity, how useful is it?

I'm thinking about respec'ing into Instigator from Destroyer and I saw Allied Opportunity and was wondering how useful is it? From what I can tell, Combat Advantage grants about 25% more damage, give or take. My understanding is that most of the time, people should have Combat Advantage against a target, or at least bosses, so what's the point of giving Combat Advantage to the rest of the group if they supposedly have it already? I'm pretty sure it doesn't stack, if it does, please correct me. Furthermore, it's only 3 seconds which isn't very long so I feel like the Combat Advantage part of this feat is pretty redundant.

Secondly, Allied Opportunity also increases Not So Fast and Mighty Leap's damage by 10% which is okay but hardly worth writing home about given their already somewhat mediocre damage for an Encounter. So here's another question I have about this feat; for most, if not all, guides I have seen, they say to use Not So Fast for AoE damage and to grant Combat Advantage to the group but why only Not So Fast and not Mighty Leap as well if granting Combat Advantage was the priority? And if Combat Advantage wasn't the priority for using Not So Fast, aren't there much better alternatives for damage?

With all that aside, there aren't many other options in the tree that could increase our damage except maybe for Stunning Flourish which I feel is so-so since I dislike using Flourish in many PvE situations with having to move all the time and whatnot. Anyways, I was trying to see the reason behind picking up Allied Opportunity and the reason for using Not So Fast and not Mighty Leap in so many Instigator builds.