You know, I did an entire post showing the math on this, and you're still going back to your 6% number, skipping several things. There's the synergy with Psychic Lock (which becomes a no-brainer if you have a psychic at-will power), the value increases when you target a foe weak to Will attacks, causing an attack to miss also allows the target to avoid status effects that occur on a hit, and the value increases considerably when the enemy uses attacks that affect multiple targets. All these things occur in addition to the power's damage capability - it's not an either/or situation.
That's true.
I was considering Heroic level. The 6% jumps to 11% at Paragon level with Psychic Lock. 1 Illusory Ambush out of 9. So, I can definitely see the value of switching Cloud of Daggers with Illusory Ambush once Psychic Lock is taken. However, there are also many other good Paragon level feats, so I suspect that it is the rare player who would take Psychic Lock at 11th level just for this (but it could happen).
And yes, against a given weaker will foe, this might be 7% that jumps to 13% once Psychic Lock is taken.
And, you are also right about attacks that affect multiple PCs. In fact, most creatures with Blasts including Dragons tend to have weaker Will saves in the MM. I'm not sure why that is. Maybe the designers wanted to give them a weak spot.
Furthermore, the value of IA is also based on how hard it is for the enemy to hit without that penalty. In an extreme case, when the enemy normally needs a 16+ to hit you, the -4 penalty of IA + Psychic Lock makes it so the enemy needs a natural 20. That seriously changes the average damage/round numbers.
Let's not talk about extreme cases. They're silly and illustrate nothing. The game is now (supposed to be) designed so that these types of extreme cases should rarely happen anymore.
As an example, the Tarrasque is so poorly designed that it is now the mathematical joke of the MM.
Fort 49, Will 32
What a freaking joke!

What moron designer put together a creature with a 17 delta between two if its defenses?
That's 85% of the D20 ratio. This totally ignores the entire "the game is now mathematically balanced" mantra that the 4E designers were talking about last year.
I can see making a Tarrasque with high Fort and low Will. But, this is just plain ridiculous (which is also why using a Tarrasque in your examples make them seem less credulous for a serious discussion as compared to using more standard creatures).
On the other hand, several daily powers last multiple rounds - Flaming Sphere can be up for an entire combat. As for whether an encounter power is appropriate - I don't know about you, but I choose my encounter powers based upon which ones have the most widespread applications, because I know they will be used in every combat.
Every combat? Wow. You and I really do play the game differently. I use up my Per Encounter power (only have one at the moment, Force Orb) maybe 1 combat in 3.
Part of the reason for this is that it's rare to get two opponents standing side by side to use Force Orb, but it's not as rare to have 2 opponents 2 squares apart to use Scorching Burst. I'd rather hit 2 opponents with Scorching Burst than 1 opponent with Force Orb (because my chance to hit at least one of them is high and I also average more damage).
However, another part of this is that Force Orb only does an extra 4.5 points of damage if used against a single target vs. Cloud of Daggers. I prefer to save Force Orb for an opportune time in an encounter, or for those times when one encounter spills into another.
Don't get me wrong. If 2 or 3 opponents are just standing together right on top of each other, I'll let Force Orb loose. But, I don't use it most encounters.
The fact that it is not a Burst is a bit of a downside to it.
It's metagaming to catch your breath and patch up your wounds after a group of monsters has just tried to kill you? If Jim the mage knows he needs a brief breather to refresh his spells, and that there's goblins in the next room, do you think he's going to charge straight in, or take the 5 minutes to go in with his full capabilities?
It's metagaming to do it every single time. It's playing the rules, not roleplaying the characters.
As an example, in our last game, we ended the night with one of the NPCs running away and the rest dispatched. He is about 40 feet away, around a corner and the PCs are convinced that he is going to get reinforcements. So, we kept initiative in the same order and are chasing after him. Even if we run into a room of enemies, they should be unprepared enemies instead of ones waiting in ambush.
I'm fine with people resting between encounters. What I consider metagaming is people ALWAYS doing that, regardless of the circumstances and the motivations of their characters. If people do this, then yes they are metagaming. IMO.
No different than waiting for your powers, health, and endurance to all come back in City of Villains for a few minutes before taking on the next opponent 50 feet away every single time. If people do this for DND (every time), they might as well be playing a MMORPG.
Let me put it another way. In 3E, we almost NEVER rested 5 minutes from one room to the next in a dungeon. So, the ONLY reason to do so in 4E is the recovery rules. Hence, anytime that someone is having their PC act in a given way every single time due to these rules and not due to the motivations of the PCs, they are metagaming. They are playing the rules, not roleplaying the PCs. Granted, the motivations of the PCs should be to rest up often between encounters. But, not every single time regardless of circumstances. IMO.