You can turn creatures into extra mana on a whim while hiding that ability on a legendary land thats unlikely to be tampered with
In addition to the new attacks and PokePowers/PokeBodies, Leveled Up Pokmon can use the attacks of the previous Level
In theory, with a Naban, Dean of Iteration on the battlefield, you only need half the number of blue mana symbols on the battlefield to make Master of Waves good (for example, in the worst-case scenario where you have just a Naban, Dean of Iteration and a Master of Waves, you'll still end up with 10 power across five bodies, which is a pretty reasonable deal for four mana, and if you throw just one other Wizard into the mix, things cat get out of hand really quickly when Master of Waves' enters-the-battlefield trigger is doubled)
Not a showstopper, but it fleshes out Gen IIIs mystical side
play lots of legendaries, but how many do we need to justify it