As per NPC Oldrak1
Oldrak: My name is Oldrak.
Oldrak: I guard this humble temple as a monument for the order of the nightmare knights.
NPC Oldrak is a Cleric (which would give him power of protecting the temple from undead (his spontaneanus speech is Beware of the undead approaching this temple!), and is involved in few quests, therefore PZ. However, I strongly believe it was also strategic decision- in old days PoH was one of most dangerous places for the players. Dragons, Giant Spider... undead Army surrounding you...Not to mention raids with Banshee and GS. Cipsoft gave a chance for the players to escape- and even with it, PoH had lots of victims (when going to hunt or explore, you would always find some dead bodies of the players- usually next to the temple).
Worth to mention that PoH seems to be Druids/Clerics residue/meeting place at some point of the past- you can find evidence in the book The Plains of Havoc2, Holy Tible can be found there3, as also the buildings like Abandoned Monastery 4:
Another temple, where you cannot respawn and is kind of similar to this would be Halls of Hope.