Because of the fact that none of the At Will Single Target powers really do a good job of controlling (including Illusory Ambush with a ~40% to ~60% chance of hitting to decrease an enemy attack by 10%, that means that it's control function actually works ~5% of the time, it's damage function works ~50% of the time), one has to assume that the intent of At Will Single Target Wizard powers is not true control. It's damage.
Numbers can be used in many ways. The value of Illusory Ambush really depends on two things - your chance to hit the target, and the target's chance to hit you. It's not a static chance.
For example, if you are running a maxed-out wizard (20 Int to begin, stat bumps at every opportunity, best possible implement for your level), you should have a pretty good chance to hit...and it only gets better if you target a foe who has Will as their weak defense. A 1st level wizard maxed on attack bonus can hit the typical brute (Will 11) 75% of the time. With Leather Armor proficiency, the wizard can have a 17 AC, or 18 if a Staff Wizard. That means the typical brute can hit the wizard roughly 45% of the time (+5 if fire beetle, +6 if dire rat), which reduces to 35% if the Illusory Ambush hits
That means the effect of the -2 from Illusory Ambush is roughly equivalent to a 16% reduction to damage taken (22% reduction in average damage times 75% chance that Illusory Attack hits), assuming you're only being attacked by one foe. Hopefully, if the defenders are doing their job, the wizard only has to contend with one foe getting past them.
Moving to the Paragon tier at 11th level, there are two Elite Brutes listed for that level in the MM - the Dire Bear (+15 attack, Will 18) and the Ogre Warhulk (+14 attack, Will 21). The wizard can take the Psychic Lock feat at this point, increasing the penalty from Illusory Ambush to -4. A +3 implement is a 11th level magic item (which can be made with Enchant Magic Item), and the wizard should have a 23 Int at this point, for a total attack bonus of +14. That's a 85% chance to hit the Dire Bear, and a 70% chance to hit the Ogre Warhulk. Averaging it out between the two still works out to roughly 75%.
At this point, the wizard's AC should be increased by +6 from 1st level (+5 level mod, +1 from Int increase), for a total AC of 23 or 24. Because the foes are Elite, they have an easier time of hitting - roughly 60%. However, with Psychic Lock, that -4 to hit reduces the chance from 60% to 40%, or a 33% reduction in average damage taken. With the 75% hit chance, that works out to a 25% overall reduction in the damage taken.
Let's go all the way to the endgame - an Ancient Red Dragon (level 30 Solo Soldier) or the Tarrasque (level 30 Solo Brute). The wizard should have a +30 to hit at this point (+9 from Int, +6 from implement, +15 from level). The Tarrasque only has a 32 Will...that's a 95% chance to hit! The Ancient Red Dragon is tougher with a 42 Will...that's only a 45% chance to hit (Soldiers have high defenses). The reduction to attacks is either -2 or -4...the Ancient Red Dragon can attack multiple times per round, and Psychic Lock only affects the first attack.
The wizard's AC is a bit more variable, here - a Wizard of the Spiral Tower with a decent Str and Con can have a light (or even heavy) shield at this point, plus Shield (or Armor) Specialization. But even a default wizard in leather armor will have a 44 AC (+10 from +6 starleather armor, +9 from Int, +15 from level). The ancient red has a +37 attack, while the Tarrasque has a +34 attack. That's anywhere from a 55% to a 70% chance to hit. Given other possible increases to AC, I'll average that out at 60%. Also, given the multiple attacks of the dragon, I'll only count half of the effect of the Psychic Lock feat for it, making it a -3.
So, with a 95% chance to reduce the average hit chance of the Tarrasque from 60% to 40%, that works out to roughly a 28% reduction in average damage taken. Even with the 45% chance to hit the Ancient Red Dragon and reduce its hit chance from 60% to 45%, that's a 11% reduction. The numbers get better if your defenders are marking the solo (either enforcing a penalty to hit or drawing its attacks to their higher AC). Still think the control aspect of Illusory Ambush is useless?
Heck, I won't even go into the ultra-AC build for mages (50 AC at level 30) that would change the numbers above to show a nearly 50% reduction in average damage for using Illusory Ambush against the Tarrasque.
Now, granted, not all monsters are weak versus Will. But that's why wizards have attacks that target a variety of defenses, with a variety of status effects.