The purity sups(blood and flesh) are relatively straight forward, but purity of the mind is a little vague. What does it actually guard against? Is it sleep or forget or bard songs or demoralize?

Purity of Mind provides a flat bonus to mind/mental saves. It's some benefit any time and a nice bump if you're already set up to pass mental save checks, but probably not enough all on its own to mean much otherwise.

It's definitely not broken, though.

I've found no use for any of them.

I have experimented with PoB vs poison and it does not reduce damage taken and you still get poisoned with the same relative ease. Ditto for PoF vs disease.

PoM does not reduce damage taken from mental attacks and doesn't seem to affect any bard songs that I can tell. I got hit with the strongest fiend when I had resist mental and PoM up and 24 int.

IMO all three are bugged.

I don't really see a difference in resisting things when they are up.

Mental jolt, confusion, bard songs. That type of things.