The problem with playing competitive games casually is that the moment one player actually bothers to learn anything about the game, it immediately ruins the experience for the other player if they're unwilling to do the same.
If nothing else, the variety keeps things interesting.
Once someone in your group starts hunting down those rare (and expensive) cards, everyone else will have to do the same or give up playing.
"I summon the Whoore Of Babylon, 70/30 flying. You're finished".