Well, I guess we have to figure out how we rank a PrC tier wise. I mean, would any full-casting PrC be tier one? If it improves a tier one's casting and gives extra goodies, I can see the argument. Are we looking at just the PrC itself, and not what it improves? Depending on how we answer these will have a lot to do with how we rank PrCs.
I can see rating the Abjurant Champion as a tier one in that it's full casting and grants some nice martial goodies (full BAB and d10 hit dice), as well as the other class abilities. It's rediculously easy to qualify for, other than having to blow a feat on Combat Casting.
I agree that I don't think I'd put Frenzied Berzerker as a tier one. It can do one thing really well, but that's about it, so by definition, I think that puts it in tier three. I agree that it's a really high tier three.
Also, certain other PrCs can bump any given PrC up a tier or so. Take Mystic Theruge. I'm not sure where to rank it by itself, but if you start using Ur Priest and similar tweaks, you can easily bump it up a tier or so.
Out of curiousity, how would you rank some of the Complete Divine PrCs like Radient Fist of Pelor, Church Inquisitor, and Sacred Exorsist? Again, I think this depends on our tier criteria. If we look at it seriously beefing an already tier one class, than that makes these PrCs tier one themselves. If we compare it to existing PrCs independatnly of the base classes, I think I'm leaning more toward tier two.