There is a difference between knowing a spell so that you can cast it, and knowing a spell such that you recognize it when you see it. You may not know Spanish or German, but you can recognize when someone is speaking it. Or. perhaps more fittingly - you may not be able to do slight of hand, but you can recognize the setup for a card trick...
I will second what others have said - knowing what is being cast at him is fine. But it doesn't mean much unless he's readied an action to counterspell or the like. It's all well and good to see the 16 ton weight dropping on your head, but it doesn't help if you can't stop it or get out of the way.
If you find the PCs doing this constantly - have the enemy spellcaster with a wand in one hand, the other free. Now, the party spellcaster must choose - ready an action to counterspell (and possibly lose the action as the enemy uses the wand), or cast his own spell (and then leave the party open to attack).
Also, what is good for the goose is good for the gander. The enemy can use the same tactics as the party.
If the character wants to use up higher level spells to counter using that feat, fine. Those same spells are then not getting used against teh enemy. The spellcaster is giving up power greater than that being used against him. That's not generally a winning proposition.
I will second what others have said - knowing what is being cast at him is fine. But it doesn't mean much unless he's readied an action to counterspell or the like. It's all well and good to see the 16 ton weight dropping on your head, but it doesn't help if you can't stop it or get out of the way.
If you find the PCs doing this constantly - have the enemy spellcaster with a wand in one hand, the other free. Now, the party spellcaster must choose - ready an action to counterspell (and possibly lose the action as the enemy uses the wand), or cast his own spell (and then leave the party open to attack).
Also, what is good for the goose is good for the gander. The enemy can use the same tactics as the party.

If the character wants to use up higher level spells to counter using that feat, fine. Those same spells are then not getting used against teh enemy. The spellcaster is giving up power greater than that being used against him. That's not generally a winning proposition.