I'm sure they talked about making the creature have a mana cost of 2C to match the original Battlemages, but they felt that being a 2/2 with two off-color abilities captured enough of the feel of a Battlemage and the cheaper mana value made it play better
Legendary Creature - Goblin Rogue Mizzix, Replica Rider Flying Whenever you cast a spell from anywhere other than your hand, you may pay
As always the list is going to include a combination of cards I like, cards that are popular and powerful, and cards that are or have been historically important to the game
Black is important to setup cards like The Infamous Cruelclaw, Felix Five-Boots and Prosper, Tome-Bound that synergize nicely
I didnt buy the argument of Lightning Bolt and Bloodbraid Elf keeping the card in check, as I played with Jace a lot in Standard and in Legacy, where both of those cards were legal yet the planeswalker ran rampant
Whether that's a bad thing, however, is up to the viewer