Yes, cards are very beneficial as it is a permanent increase to your stat(s) immediately after using them.
If you know you're going to be only a magic user, such as Mage or Warlock, you may want to hang on and add all the Int/Stam/Def cards and perhaps consider selling the others you get for some extra cash.
Likewise, if you know you're going to only be a physical attacker, you may take all the Int/Wis stats and dump them off for the magic users in the AH for that extra cash.
Or, you can keep them all for yourself, it's of course your choice.