I have never even once through my 3 game plays of FFIII, switched any of my characters jobs to a geomancer. Not because I don't think they aren't useful (You actually need them in FFV in EXDeath's castle), but I leaned more to the fundamental classes (Thief, Warrior, White mage, Black mage, Monk) throughout most of the game until the I get the Water and Earth crystal jobs.

When I get a DS and replay it toward a completion file (All jobs mastered, unlock Onion Knight job, Beat Iron Giant without the 4 Onion Knight or devout/ninja teams), I will end using the geomancer.