Guess I am in the minority when it comes to Enemy Cards.
I love to use them, sure they are free for Riku so I can use them there practically penaltyless, but even for Sora I always make sure to have the following Enemy Cards: Vexen, Marluxia, Neoshadow, Oogie Boogie, (and post game) Lexaeus to quickly finish Heartless batches.
I lost my cartridge for the GBA version shortly after 100% it,which broke my heart, but I clearly remember having three Neoshadow Enemy Cards in my deck as well as two Megalixers.
When a battle started, mook or boss, I would start out by playing my first Neoshadow, then use up all the cards in my deck and reload manually then use my second Neoshadow, waste my deck again, if the battle was still not finished I would use my first Megalixer and the use my last Neoshadow. Then I would start using my other Enemy cards. Usually Marluxia first for double the amount of sleights.
Even though I only had the game for just under a year before I lost it, to this day GBA Chain of Memories is still my favourite GBA game, and I sorely miss it...
I have several times tried to recreate my GBA deck in Re:CoM, but that game has just never given me the same kind of satisfaction, forcing me to give up after having collected 2 Neoshadows.
The more I think of this game, the more my old feeling of letdown with KHII after the awesomeness that was CoM comes back to me. I guess that is what happens when you start to remember something about a game centered around memories lol.