Going over the three insane careers in the core rules I noticed that they all had the Piety skill allowed but not Invocation… After going through the rules sections on Piety and faith I have found nothing to explain this oddity.

The skill only seems to deal with curry favor and fuelling blessings and while initiates and clergy could advance through the insane careers I got to wonder about wether or not a character with Peity could invoke the basic blessing 'minor blessing'.

Additionally each of the three faiths covered in the basic rules has a blessing that does not require any skillcheck, so would a Zealot dedicated to Sigmar be allowed to aquire and use the 'Gritted Teeth' blessing?

The blessings require no check and thus invocation is not strictly needed and the piety skill would allow the zealot to curry any needed additional favor to fuel the blessing.

The available blessings such a character could gain access to are usefull but to my mind would not in any way overpower such a character.

Naturally Shallyan Flagellants and Witch Hunters seem like a very strange things as Shallya would do what is possible to cure such an individual rather than grant blessings however if such an individual was on a holy mission against Nurgle…

Also the Sigmarite blessing 'Penitent Zeal' requires a Piety check to activate and not Invocation… typo or intentional? is it available for non invocation holders?

In short is the Piety skill option there only for clergy class diversification or… ?

In short I'd say that Piety is the knowledge about the gods of the empire and the practices associated with it. The key beeing "a blend of knowledge and intuition on what will be pleasing and approprita to the gods". In my games it can be used much like education or folk lore when it comes to knowledge about the gods and the mortal practices surrounding them, even though folk lore will give a general knowledge (I at least assume that most people in the empire have a bit of faith in the gods), piety would give you specifics.

It's also a good way for a Zelot to "prepare" for the initiate career.

The "Sermon" action card I believe uses piety, indicating that one could use piety when holding a speech which involves the gods somehow. For a Witch Hunter that might something like rousing sermon for the pesants in a village to join the Witch Hunter in the burning of a Witch or Chaos Mutant.

Lastly, I'd say it should be used as a roleplaying tool and "fluff" for the character. I think it would be rather wierd if Witch Hunters and Zelots did not have Piety as they are (overly) pious characters in general. That this skill is avaliable to them reflects that.

Oh and I do allow my Zelot player to use blessings that require only a Piety check, like Penitent Zeal for example.