Advent Ultimate can take enough turns because trash mobs exist and I can definitely see using Jack Frost over the 45% water eidolon as a main for some battles just because the effect is just that much better. The 45% water eidolons battle effect is literally the passive of Jack Frost, a fire resistance buff. It's going to suck when everyone on my FL switches over to the 45% water eidolon.
Regardless, it was just a way to look at how it could be useful. Not everyone is as strong as you are.