I wouldn't call it "mandatory", but it'll probably end up on the bad end of nerfhammer due to how good it is now. Almost invisible, weapons ignore armor, voodoo resistant, and now has an all-seeing eye.
It's mainly good because of the invisibility which is now required to deal with smoke ignoring enemies. It's either use ghost or facetank, and if you don't have some seriously upper end armour, the latter is a pretty bad idea.
Ghost knife/beam seem amazing up until the point you realize you can have the former on any armour, and the lethality of the latter is readily matched or surpassed by most late game weaponry (especially given the beam's insane TU cost, and energy drain that I feel was needlessly increased). Meanwhile no nades/medikits/utility/stunning items or equipment flexibility for you which is a massive drawback. Overall I think it's in a pretty decent balance space (though the Ghost Beam should probably be either a little less prohibitive to use, or a little more powerful), because I'm running them strictly as scouts rather than the core of my force (that would be Witches).