in fact lets expand upon what mage can do everything
sch can not debuff without an apropriate SJ no dia bio poison para blind all they get is sleeps and break thats it
whm is a healer (if they are not focusing on that find a new one)
bard buffs cant cure cant DD no debuffs outside of elegy and requiem
cor buffs, has no real debuffs of its own can DD but can not cure
RDM used to be the jack of all trades last time I played with one in a relevent event it was 2010
GEO unresistable debuffs with many unique ones that all stack with other jobs can DD with T4 nukes and RA spells as well as buffs that are shared only between it and cor (well placed in tank PT for refresh bubble to the white mage or with APROPRIATE SUB up close in the frey)
BLM DD cant cure can do blind break and sleep

which of these mages can do everything you want geo to do

you want it to buff debuff support with cures and rival all other mages

all its buffs unlike say a whm and a sch stack I can have a geo defence boost on the PLD and a whm protect I cant have 2 protect 5's on the pld

all its debuffs stack so I can have dia 3 and geo defence down on the mob at the same time or I can even bio the mob and still have a defence or even attack down on the mob.

Why is it that you think geo cant play in a team as well as other mages or that its stuck bound to certain roles that it NEEDS to fill to be useful.