If you pay two generic mana when tapping Three Tree City, you can generate a ton of mana for any color equal to the number of creatures you control that are a certain type
And while having the option to choose the most useful one is nice, it's not enough to make it something that makes the cut consistently
She provides protection if you can enchant your opponents' board so they don't attack you, but even better, a potentially game-ending ability
Also of note is the penalty for losing: If your Mega faints, your opponent will get 3 point, instantly winning the match
Then there's blue's connection to delve, flashback, and harmonize, all of which get cast or activated from the graveyard, so much that you might call this Diver a Driver because it's a key piece of mill and card advantage engines