Finally, Rogue is getting a decent 4-drop: a card slot, which was heavily ignored by Blizzard in the past.
Every longtime Hearthstone player should remember the Ambush spell that had similar effect, but instead of putting three spiders into your deck, it gave them to your opponent. It was a fun card, but it was flawed since you couldn't control the opponent's draws.
Fal'dorei Strider, on the other hand, puts spiders right into your deck. And if you have a card draw engine ready for the next turn, you can easily bring two or all three spiders on board and thus acquire massive advantage.