This ability is so powerful because it's meaningful at any stage of the game, whether you need more mana to ramp out the rest of your hand or more cards to finish off your opponent
Mechanically, there are few things it can do that other colors also can't, making it feel like dilute and less focused than blue, black, red or green
Survival of the Fittest does the work of two or more cards in its own little combo, and it only requires 3 mana to get going
Granted, its different kinds of Energy so youll need to come up with a solution to put those in your hand
With multiple Greninja BREAKs on the board and some energy in your hand (Fisherman, anyone?), you can do some serious damage to your opponents Pokemon to take some easy prizes, and thats before you even attack
If a player is getting too far ahead and controls Xantcha, other players can scheme to use their mana to knock the leader down a few notches