It's a cool spell, but the "can't be replaced" clause doesn't add much. Consider if this gets revealed on your chest piece: you let it get destroyed during upkeep, then put out a new chest piece during your quickcast (or deployed from Battle Forge). If your opponent has initiative, then there's a brief opportunity during opening quickcast for her to get a cheap shot I before you get a new chest piece - or to put a Nullify on you to cancel the new chest piece - but other than that, it isn't much different than being able to replace it by casting over it.
How many chest pieces do You have in your spellbook? 4?
None of your business.
What I meant was, if you have 1 chest piece equipped and at least 1 more in your spell book (which isn't unusual, in my experience), then you could let your first get destroyed by Curse Item and cast the other during the quickcast phase following the upkeep during which it was destroyed... which isn't much different than replacing it during a quickcast phase. Perhaps the only difference is you can't pay the 2 damage cost, then quickcast new armor during your opening quickcast, preventing your opponent from having an opportunity to blast you with an attack spell during their opening quickcast on a round when they have initiative when you're temporarily down a chest piece... but taking 2 direct damage to maintain +2 armor isn't generally better than not taking 2 direct damage and losing that 2 armor for 1 quickcast phase (although there are certainly cases where it is better, like losing your Dragonscale Hauberk and then taking a Fireball to the face, or losing your Chitin Armor and getting splattered with an Acid Ball/Blast).
Another use of Cursed Item is if you have enough Corrode to bring your net Armor to 0, a Curse Item could be used to effectively block your planned armor-swap, messing up your turn (e.g. preventing you from using a planned spell). That's pretty nasty, actually.
And in the case of slapping a Nullify on you when you have a Cursed Item, they could still do that during the opening quickcast with initiative regardless of whether or not you are allowed to replace the equipment, so that's a wash. So... not being able to replace the equipment while cursed doesn't really change much from an in-game perspective.
However, it is highly thematic. There are lots of RPG's where, if you equip an (unidentified) cursed equipment, you can't easily get rid of it (e.g. armor-swap). Hopefully we'll see a piece of equipment itself that is cursed (e.g. has some negative stats or deals damage during upkeep) that can be cast on an opposing mage that can't be replaced and must be dealt with in some other way (e.g. Dissolve or Crumble or Explode). I'd love to be able to deploy such a cursed equipment on an opposing mage by a Battle Forge!