I think making these checks to give monsters tactical information about class abilities are a bit too much to most monsters. We would never allow that to PCs.
The abilities a monster might have because of their training are not differentiated from other powers. So a knowledge check on an Orc, for instance, will give you a good idea of what capabilities he has. Knowledge of a monster is not relegated to anatomy. Look at every monster knowledge blurb in the MM for monsters with a society; look! Cultural and societal information.
Now, let's say I asked you to tell me about Persians during the battle of thermopylae. Would you tell me of their physical descriptions? Or would you tell me that these guys had an elite group of warriors called the Immortals? And does knowledge of how to fight greeks not include the word 'phalanx' in there?
If you can get cultural information, you can get information about likely battle tactics, abilities, and forms--especially if you have proffessional level capacity in battle, tactics, and abilities. An adventurer would certainly apply here.
A capable combat-ready monster has a chance to see a combatant, and go 'Well, let's see. He holds his sword in a defensive manner, with constantly harrassing attacks and feints. If I turn my back on him, he'll use that opportunity to attack me.' This isn't mystical crazy secret stuff here, this is someone who understands fighting trying to survive, and possibly trained in how to counter other fighting styles.
I cannot -possibly- see how it is unfair that players can use their plentiful skills but monsters cannot use them in -the exact same way.- In terms of skill use the players already have their advantage. If, every so often, a swordmage goes 'Wait, how did he know about that swordmage aegis?' it's not completely mindbending to say 'Look, this guy happens to be the head magician for the kingdom of Magiciania, a place that has a standing army of mages, sorcerers, spellcasters, and yes, even swordmages. I think that with his vast knowledge of arcane mysteries, he might have seen your spell a couple times, or even be able to deduce what it does with a simple check.'
And when the Avenger goes 'How did that guy know about my Oath of Emnity?' sometimes 'He's the high priest of Vecna' should be enough of an explanation.
The alternative is hitting every single monster the players come against with
the Idiot Ball which is completely unbelievable and is NOT good storytelling.
Should -every- monster be omnipotent? Hell no. Should the occasional monster know what the players bring to the fight? Sure. Should all monsters be ignorant? Hell no.
The middle ground is the best way to adjudicate that, and if that means rolling some dice, a knowledge check is a fine way to do so in cases where knowledge is plausible but not certain.
'But it's not fair!' is a kneejerk reaction. Prove that it is not fair. And while you're at it, explain why certain monsters -have- knowledge skills if they cannot use them? What is the point of giving them these abilities? Isn't the point of 4e edition monster design to give them stuff they can actually -use-?