The interesting thing about Graverobber, as opposed to other "deck cannibalization" cards, is that it doesn't help with early-game thinning at all. Contrast to Salvager, Apprentice, Bishop, Remodel, Expand, etc. which can all do double duty, either thinning or to cannibalize for endgame or even midgame payload. The issue is when to start cannibalizing one's deck (i.e. trashing good cards such as engine pieces or payload to advance the endgame) and this really screws with the the game tempo and can be difficult to get a feel for. But at least with those other TfB cards, there is usally a benefit associated with the thinning itself which lowers the opportunity cost of these cards.
So when buying a Graverobber, the intention really is mid- to endgame cannibalization or to recover fodder from other TfB cards or trashing attack. I believe that the skill involved for when to pick one of these up is actually unique than for any other card. In some ways, it's like picking up a potion when the only Alchemy cards are Vineyard or Possession...it telegraphs the type of game you're going to be playing and can therefore be decisive if done properly or set your opponent up to crush you if you do it too early, too late, or when your opponent's deck is better built for that type of game and was just building until you commit to it first.
