@desolutionist agreed, basically when dark petition is bad grim tutor tends to be slightly better, but when dark petition is good grim tutor is a lot worse.
@Ten-Ten you should probably give the card a try then, it's a lot better than it may appear. It's more versatile than ad nauseam and able to serve the same function by fetching for necropotence. I like to have at least one petition with gifts since its pretty good in a number of piles and bridges the gap between gifts and will if you already have it hand.
The only real weakness of the card is that it's pretty bad against cards like Rest in Peace, but even then it's not totally dead (i've won a surprising amount of games by paying 5 mana to tutor for something like pack rat/bounce spell).
Most blue decks are fighting combo with Leovold/Nthief, null rod and pyroblast at the moment. As a result Cabal ritual + Dark petition is pretty strongly positionned right now. It's also great to have YBargain and DPetition in the same deck since they work well together and each one of them is resilient to the hate that targets the other (Petition gets around Card draw hate and pithing needle effects, while bargain gets around gravehate and flusterstorm)