EDIT - AD posted while I was typing, so that might make some things I've said obsolete
In response to what has been said, I consider the "This card has 0 hp but cannot die" clause to be avoiding the problem. Aside from all the funky business it brings up with stuff like Otyugh, Immo, Maxwell's, Catapult (wat?), Freeze + Shockwave, it breaks a fundamental rule of the game. Creatures with 0 hp die. That's it, the end. A card that can survive with 0 hp doesn't make sense, and the concept of negative hp is illogical (I know negative attack is a think, but its easy to think of negative attack as healing, but what does negative hp mean?).
The "Come back to life after death" might mean the same thing as "This card cannot die" but its a better way of doing it. It's still a work-around of the 0 hp concept, and it still basically means that once the damage hits the field, it won't be going anywhere any time soon, but it's a better way of doing things. If you buff a card like this, and it dies, you get it back in its original state. However, if you buff a "cannot die" creature, that buff stays (especially since there's no way to remove attack currently). Of course 0 hp creatures that come back every turn have their own problems with death effects, but that's to do with balance, not illogical concepts.
Given that I didn't even think of the "cannot die" concept, and that I considered and shot down the "comes back after death" idea, I narrowed down how I wanted the card to work to two possibilities. I considered making a creature with the mechanic "If this creature dies, do ____", but a creature like that could (and probably should) just be a spell instead. It fits better in most cases, and rules out any Fractal shenanigans that might occur.
So I went for my other idea, make a card that thematically would have 0 hp. The idea being it's a lost soul that wants to go to the after life, but if you keep it trapped in this world, it'll start haunting the place. I might not have made a card that is totally hinged on the 0 hp thing, but I made a creature that has a thematic reason to have 0 hp, like Spark and Phase Salvager have a thematic reason to have 0 hp (seeing as these are the only 0 hp creatures in game, I'm going to keep making this comparison). And yes, the Poltergeist could have 1 hp and still kinda make sense, but so could Spark and Phase Salvager, they just don't.