eidolons arnt very useful. i,d say only atomos gravija like move, odins instant kill, bahamats 9999 damage are the only real good 1's. i just use them if garnet has a free go.

ark just looks super cool so i use him to sumtimes. i think it was very silly not to allow ark to do at least 50000 like eden.

still, they arnt as useless as ff6s espers. then again, they were a huge part of ff6's story and gave u stats and magic... ok ff9 eidolons suck.