The reason for the three stoned party members is for two reasons.
1. They take damage for you instead most of the time, so you will be able to go longer without needing to heal.
2. Sleeping bags are cheaper then tents and that is all you will need while leveling up 1 member at a time.
Actually there is other reasons,
3. Your one and only fighter will reap the rewards and get 4x the Exp the usual cause the others are stoned.
4. Having one exp egg means you will only need to equip that and with one member you will get 8x the usual exp cause it is not split.
5. You will have all the espers and equipment at top tier while you slash all the enemies. This will allow great use with little to no hassle of changing espers between all the members.