The biggest advantage of having low level characters is they can get more out of the GF Catuar's stat bonuses. I had three party members at level 15 when I got Catuar. The other three I had been using more were around 30-35 or so. I then leveled each party member up to level 100 using the stat bonuses.

While the bosses are less hard at lower levels I like having all the junction abilities for each GF, so I do a lot of leveling up.