When does a Zeus X make in-universe sense? The only situation I could come up with is in a special operations unit with a strict limit on dropship 'mech-bays
It's a good mech on the tabletop; not the equal of a Timberwolf, but in the same ballpark. The problem is the 400XXL driving the cost to stratospheric levels. You could buy literal battalions of bug 'mechs for the cost of one.
The early Zeus Xs are merely prototypes, experimental things, tech showcases. I figure they're there to showcase Defiance's technological prowess, as such they make sense in-universe as marketing pieces.
By the time Zeus X4 rolls out, the XXL engines are relatively common in-universe with several production-grade 'Mechs using them (Savage Wolf, Dasher II, Warhammer IIC variant, Prey Seeker, Vulpes, Wulfen, to name few) and tournament legal rules-wise. They do retain their cost rules-wise, but presumably those costs do not hold true in-universe (if they did, 99% of BattleTech wouldn't work at all).
Personally, i'm rather worried about the 'Mech's design and purpose than its economics. Reactive armor isn't bad, but composite structure is dubious, especially with something as fragile as an XXL engine. Integral command console means the 'Mech is a command unit and thus likely in limited production only, it most certainly means there is little purpose in having multiple ones on the field at once.
But perhaps those features combined with its weapons loadout make it tougher than it looks at a glance. Its armament is geared for long-range fighting, while not great compared to Clan weapons, it is decent vs Inner Sphere, especially with its speed. And it has fortunately low damage output, meaning it is limited threat in itself. Basically a mobile command 'Mech.
But i'm not sure that idea is that useful, nor why not just pick some more traditional command unit, unless the Zeus X4 is intended for cavalry formations.