Manditory Card Sleeves?

What do you guys think about mandatory card sleeves, maybe not mandatory but makes the game significantly easier.

Why?

1). Team differentiation: in my game there are times when you can control or steal enemy cards for entire games sometimes. Some decks can be quite similar so two identical cards on the field controlled by different players can get annoying.

Also some cards are faced down and can be seen by you but not by the enemy, there are times when face down cards from the enemy can be put down near your face down cards. Which one is mine?

2). Cards with cards: Some cards will have enhancement cards like Armor that they carry with them on the board throughout the game (or until an enemy removes it by attacking or landing in the same space as the card).

I find it annoying to have to "babysit" the extra card. I was thinking with sleeves you could just insert the Armor card in the sleeve so it almost acts as one card. Once an enemy card attacks you you just remove the Armor card from within the sleeve while leaving the actual creature/unit inside the sleeve.

Thoughts? I'm sure this has been done before. I was just looking for input as to how doable or solid the idea is.

Thanks,

Tbone