the prolific mana base of slaver obviously makes a draw x spell attractive, and read the runes fills the graveyard with welder targets at the same time. it is an instant, and it only gives 1 mana drain mana if they happen to get it. the only con i can even think of is finding space for it, or perhaps the fact that at levels of mana it is strictly worse than thirst for knowledge, but there is some redundancy in most builds that could easily be replaced with a draw engine of this power level. it seems to be an ideal way of refueling if you happen to run out of gas at the middle stage of the game, filling a niche very similar to that of the draw 7 with the bonus of being an instant (and not being restricted), but with a higher power level at this stage of the game than thirst for knowledge.