So, if you have them both in play, Sephiroth can sacrifice Aerith for his fourth trigger, transforming, and then reap the benefits of all the +1/+1 tokens Aerith had built up from his attacking
You can generate a colorless mana without any downside, or fix colors while casting a legend, not to mention the cant be countered clause
One look at the Magic: The Gathering release schedule shows that this isn't going to change anytime soon, either
Finally, the last ability puts Gandalf on top of your library
With a very "blue" scry-like ability, this card doesn't need to flip to be useful late in the game, but is incredibly helpful when flipped early in the game as well
When you run this in a Dimir colored deck, toss in mana rocks like Fellwar Stone and Darksteel Ingot that tap for more mana than just Dimir to cast them