Typical. People have been asking about this since early beta, and yet there still is no response from FFG regarding Purity Seals. How hard can it be? What are they waiting for?
In TT, the Purity Seal allowed an IG Plasma Gunner to re-roll 1s on to-hit dice, provided extra movement for the zealous Black Templars, and allowed Space Marines to roll an extra die for their Fall Back move and choose the two they wanted to use.
Perhaps have them provide a +5 bonus to Fear Tests, and reduce Corruption from Warp Shock by 1 (to a minimum of 1). Or, allow PCs with Purity Seals to Disengage as a Half Action. Or, allow a PC to re-roll a BS Test when a weapon Overheats.
It's actually ironic, considering Purity Seals never had a consistent effect in TT; in one incarnation of a codex they'd be in and the next they'd be out. They must not be as important as one might expect. All the artwork depicting Confessors, Priests, Banishers, and Sororitas draped in a multitude of Purity Seals…for no reason other than visual asthetics.