The thing about defenses is that you can only use a maximum of one per attack, and most defenses can only be used once per round. As far as I know, there are no defenses usable by a mage that can be used more than once per round.
Considering this, multiple defenses on any creature are better off for solo and few big/buddy strategies.
Mages with more creatures aren't going to care as much about defenses on their own side because they have more creature actions to attack and guard. I think most mages that aren't solo or few big will either put a defense or two on an important creature or use a creature with an innate defense.
Also, defenses are not that significant a problem for the warlord. He has sniper shot and perfect strike and also Morning Star coming out, and those are just the in school options.
Also, if this is an incantation I think it's effect should be just until end of round the target creature cannot use defenses or, destroy target enchantment that gives a defense. Why prevent a creature from having any defenses ever if the defenses aren't that helpful for that creatures survival, or if you're not planning to attack and destroy them over the course of more than one round rather than just one. If that were the case, it world only be useful against a mage or an important non mage creature.
However, an incantation with a permanent defense removal effect that can target a mage would be WAY OP IMO. It would only be useful against a long lasting creature that used a lot of defenses, in which case either you're better off stopping that creature with a bit of position control or it's the enemy mage.
It would be extremely situational since it would basically only ever be helpful against a mage that relies a lot on defenses, but when facing that mage it would neuter them.
