Earth has some things that other schools don't.
Poison cloud + portcullis is a very good method to deal with any monster.
Poison bolts are really good against Uggardians as they usually hover left to right and thus stay longer in the poison cloud than most enemies.
Poison attacks are also good if you are pitched into a corner / alcove and HAVE to fight your way through the enemy. If it doesn't move, it takes insane damage in the cloud.
Also mana costs. I loved to use my ice bolt to freeze enemies, but it seemed to drain mana so fast I usually cast more poison bolts for pure dps.
Why earth is so hated? I found it to be really good dps output school throughout the game.
Not sure, but I don't think there was a similar item like Shaman's staff for other schools, also poison shield is also nothing to be laughed at.
Eath Magic
Re: Eath Magic
There is an Orb that boosts fire later on, but I'm not aware of any boosts to Ice or Air.