I got an agnostic's collar, then defiled a couple of altars and got at least one piety bonus for it (not sure if I got one or two) then every subsequent defiling (in the naga prayer room) didn't get me any piety. Without an agnostic's collar, defiling two of those altars definitely got me two piety bonuses, I didn't try for a third because it was a critical endgame situation. Seems like a pretty worthless item if you can't defile more altars and get the benefits, especially taking up a whole large inventory slot and at the price! And I don't see anything in any of the texts saying you won't get piety for defiling after the first couple, which is why I think it's a bug.
While I'm here, I'd also suggest that all the class intrinsic "+X% bonus damage" are pretty misleading, because those effects actually only add X% of base damage. This is okay once you're used to it, but it's frustrating and confusing as a new player, because that's sometimes only about half of what you might be expecting.