User blog comment:ClariS/Why Xerath's Passive is Bad/@comment-163.191.194.60-20130320135709

His passive is perfectly fine. Lore-wise: He is made of energy, not matter. He absolutely should take less physical damage than anyone else. If you want to bring his armor sarcophagus into this, according to his lore, his power was (and still is) degrading the sarcophagus' bindings. It is only natural that as he accumulates power (read Ability Power) that the sarcophagus would get weaker, and it's ability to bind him would become even weaker, revealing more of his energy body and therefore taking less physical damage.

Game-wise:  Every passive has a use, and Xerath's is one of my favorites. It changes my item dynamics for picks, it does help against physical damage, and it allows me to play differently. In a teamfight you will almost never be at max range, you will either be too far away or you will have to risk gap closers. Do what you are there to do, make sure you're in range, risk the gap-closers and do bunches of aoe damage, this passive helps this massively.

"Me-wise:" You should want diversity in passives, last thing I want is every champion of any type having the same passives with slightly different wording. Instead there are things like Xerath (long range with tanky passive), Orianna (supportish mid with psuedo ADC passive), and Dianna (AP assassin with an AD passive).

tl;dr: His passive is fine, it fits lore, fits fantasy physics, and has a use in game.