Dunno about that. I’m perfectly happy playing an Inquisitor without the seal. In fact, on my last Inquisitor (a gun+shield Purifier based of mad_lee build) I moved all 13 points from the seal into, IIRC, Vindicative Flame, Bursting Round and Deadly Aim for an even more destructive build.
I don’t think the Seal makes or breaks this mastery. But yes, it’s crappy. In fact, the reason why I moved those points was exactly because of the things that are being said here: I couldn’t find a valid and consistent use for so many points.
EDIT: Just went and took a look. It was just Vindicative Flame and Deadly Aim.