Re: What was the rationale in making pretenders un-Blessable?
 

I don't agree. Pretenders have serving mortals for that (blessing, instilling fanaticism). Priests or whatever... They don't put their hands into the "manual work" of their cults and practices. They are the object of cult, not a minister of it.
IMHO.

Re: What was the rationale in making pretenders un-Blessable?
 

Exactly Tifone, which is why they shouldn't be given priestly power. That doesn't mean that that symbol is worthless to the people who behold it.