The only cards one should partially upgrade are gladiators, because they can't be sold or rented out. Great tips for the newbies, Achim!

Here's another reason why one would want to keep versions of the same card. Let's take Furious Chicken, for example. I have one in GF, one normal upgraded to level 6 (rented out), and a few others, I think all at level 1. Here's the difference. Normally I play the GF, for the GF boost. But if the ruleset doesn't allow melee cards, level 1 chicken can be played, because it has no attack. If one upgrades a card using all available cards, then these opportunities might be missed, when a lower level card can be played where a higher level version of the card cannot.