Is there any use for the skill piety outside of the cast blessing business?

if not, why can a zealot have the ability?

I think it can also be used socially for checks related to religion - like preaching to NPC's (and thus making them do as you wish) or for making you knowledgeable in religious matters.

Iffo said:

This is how we've been using it.

For example: having a Sigmarite recognize and understand a symbol or ritual from another cult.