"To reliably get Darmanitan's HP to 50% is very difficult, and at that level, it is easy to be KOed. The only really reliable way to do it is to use Belly Drum, which is pointless as in its Zen Mode, Darmanitan is seriously slow and has an extremely low Attack stat. As well as this, lowered Speed does not aid Zen Mode Darmanitan's sweeping capabilities, and so Darmanitan should stick with the abuse of Sheer Force and physical attacks. It's not really clear what Game Freak were thinking when they thought up this ability."
The one possible saving grace that Zen Mode Darmanitan gets would be increased defenses...however, the addition of a psychic-type on the transformation hurts it far more than the increased defenses help. The swapping of base attack stats also means that you'd have to awkwardly split EVs to get the most out of it in both forms somehow.
While it does benefit from a recoil-less STAB move in that form in Overheat as well as a new toy in STAB on any psychic attacks, one could argue that the mind-boggling power of a Sheer Force STAB Flare Blitz (with LO recoil negation!) is the main reason for using Darmanitan. If you're willing to take the plunge and try to fill this hole, be our guest.
I've answered this somewhere else, but anyway, the reason for it is because just like Basculin-B, Darmanitan-Z registers as another Pokemon forme (a la Wormadam), and therefore is in the database as another Pokemon in the Dex. IDK how hard it is to get it out of the database (which only Arti has access to atm), but until then it will stay on the Dex.
Giving it a joke analysis would be pointless since it's going to be (hopefully) deleted at some point in the future, and for now it's listed under Limbo in the Dex, along with Meloetta, Keldeo, and Genesect, so it should imply that there isn't really anywhere to put it (and why you would want or need to use it is beyond me).