if you are leveling past 200 its mandatory. there is no reason to have it on. even if u have to take off a 5th job node in its place. HS should be taking 1 of your slots.
even level 1 HS is useful. most 5th job skills have a heavy Cooldown and for merc specifically i already know 2 of the 4th job skills you start with are not that neccessary.
you can keep your pony cus the mobility and power on that is worth the cooldown.