RE: I play over 2 months and never used half of available cards

I do use both fire cards. If you play for example a ruleset where all melee have sneak. If the enemy has attackers with 1 melee or ranged, then Serpentine Soldier will stay at the end of the attack, while Cerberus will fall. Also Serpentine Soldier has a higher speed, and so also a higher chance to dodge.

For the other two you are right, these were also not introduced with the same card set. Unfortunately Earth Elemental can't be used in standard ranked games from the release of the CL packs.

A similar comparison is currently with Sabre Shark and Pelacor Bandit. Thought an Alpha edition Sabre Shark will give you a nice bonus for a won game. :D