In fairness, Animate Dead is fine without the Earth Magic skill.
I think the reason earth is truly broken is slow. Makes no sense that a lvl 5 can cast extreme slow on an entire army and just range-it away.
Town portal was clearly put in there to patch the terrible expeirence of spending 2 hours to move a heroe across the map. But no reason for slow to be so imbalanced!
I find it interesting how later iterations of the game fixed the Mass Slow problem, but (perhaps with the exception of the King's Bounty reboot) generally weren't considered as fun to play.
Mass slow (seriously overpowered, especially against AI), mass shield, mass stone skin - 3 of the best 1st level spells belong to earth magic school. Not to mention town portal, anti-magic, resurrection and the most powerful offensive spell - implosion. The only case I occasionally skip earth magic is when I play barbarians, where air magic (mass haste) is more important and game is expected to end during the first month.