How? Ice is very different from fire. One is about heals and direct control with little damage while the other is raw damage and good AoE. Aero has some control and lots of utility. Earth is DoT's, healing for undead, defense and some controls. Summoner is all about the infernal and he is definitely worth it. Necro & Poly are various forms of utility. Poly has good control while necro has a good meat shield, self heals.
If anything, one skill branch in this game is like 3-4 different classes in FFT. In other words, this one has a lot more variation.