There are only 3 situation for me using 6SS though.
1) After LB3(when mnk is the only melee here), even fresh apply demolish/snap punch, the animation takes too long and mnk won’t have enough time to refresh GL.Form shift is DPS lost here so perfect balance might be best solution but if PB down for any reason, I use 6SS as LAST RESORT.
2) Incoming transition so we use 6SS+ tornado Kick, but Mnk need to make sure that Anatman can refesh to 4th GL in time before target become available or else just 6SS without tornado kick in short transition.
3) In dungeon,either last mob or boss if at 1% hp then I use 6SS here for looking cool.

For my thought,I think this skill is both useful and useless at the same time.
Even without it we can manage to sustain our GL without problem.