You mean Recycle?
If priest ever becomes an issue, quest mage will be there to clean it up.
First off, I'd like to say that this card is another wrong step in direction for Hearthstone. Random card generation is just super unhealthy for the game, especially for a class like Mage with such high quality cards. This mechanic is the least fun and interactive, since neither players have control over it and it can feel really unfair to lose based on a random outcome, applicable to the player who plays this card as well. It's unfair to say that heroes like Gul'dan have bad quality cards since his hero power is very strong (which is undeniable) while classes like Mage is pampered with cards like these. Discover mechanics for random cards is still somewhat acceptable since you at least it isn't entirely random, but pure random card generation should just go extinct after the next (Un'Goro) rotation.
Now, the actual card itself. IMO it well definitely see play in control mage (running Jaina and certain elementals like Fire Fly and Tar Creeper). It's worth mentioning that a pair of Fire Flies are enough to activate it's final form. It may see play in Exodia with a Fire Fly tempo package, but that is highly unlikely since it's not easy to free up more than two more spaces in such a deck, even if Cabalist's Tome rotates out.
you are forgetting at least the pack, so you gained 450 and 150 golds, besides you also get dust, golden cards ecc...
Arena is the most rewarding mod in the game without a doubt, you can get epics and legendaries but thats pretty rare.