Well (and I apologize for highjacking the thread), I think the root power-creep cause is that Dark Heresy was designed as a gothic horror game for low-power-level characters, esp. in the beginning (so characters start off as more or less average guys facing an Evil Universe Far Greater Than Them)
A lot of the player base was not happy with this. (I'm sure we remember all the conversartions about how starting Acolytes are incompetent.) A lot of people don't want to play Dark Heresy as Space Call of Cthulhu, but as the pulp action genre that you get in Abnett's books, for instance. So you get the power creep that started in Blood of Martyrs, as well as the linear, almost video-game-like, combat-oriented structure of the Apostasy Gambit.