Okay, now compare the gods from the alpha too
FYI: Petition is there to balance out JJ's lack of piety-decreasing actions. The cost to worshipping JJ is you have to be flexible to deal with the things that get thrown at you. Other gods all have negative piety hits that they lob at you, yes, you can avoid them by playing within their rules - just like you can mitigate the impact of the afflictions that JJ sends your way. Put it this was: JJ's piety tax is explicit in a boon, the other gods have implicit taxes on your actions.
Tikki's poison is considered too. It's there if you want to convert out later OR if you know that you're going to have to handle multiple hit enemies or are forced to regen-fight. It'll help you take less hits AND give you piety to help mitigate the piety tax.