Well, this problem has always been a bit difficult to pin down.
On the one hand, the rules for ghosts and such in D&D and Pathfinder have always been mostly clear, if overcomplicated with all kinds of specific cases and applications. On the other hand, these rules are mostly a failure, in my estimation. While they do work, the rules for incorporeal creatures add zero flavor, richness, or realism to the game. It’s worth noting that Paizo came out with the Haunt rules so that ghostly events could happen without evoking the spirit rules.
Instead, we have ghosts and wraiths taking partial damage from spells and magic weapons and floating around on another plane of existence. There are even rules covering the inevitable “hide within the stone wall and attack people” rules exploit. These mechanics are fine if you just want to play extended encounters and tactical games, or find all kinds of ways to exploit corner cases and creative interpretations. However, if you want your spirits to strike fear into the players’ hearts or evoke an eerie mood, they need to interact with these monsters with a phrase other than “Hey, are you carrying that ghost touch dagger or did we sell it? This fight could be annoying.”
I haven’t yet worked out a system of mechanics for incorporeal creatures that evokes all the right feelings. I’m still experimenting. However, the first step to making a better rule is to identify what bothers us about the old rule and figure out the goals for a new one.
-Max Porter Zasada