Instead of sacrificing a creature and getting , for the Invasion version, you get the more fitting one mana of any color. While the reduction in amount of mana received might sound like a drawback, colored mana often goes a long way further than colorless
Tapping for a red/blue and blue/black respectively, you can guess how Grixis is the best color combination
Effects that modify the creature's power and/or toughness without setting base power and/or toughness will apply to the creature no matter when they started to take effect
Or maybe I should be calling it Mardu Justice League
as well, but the cost is so high that even the best top end in purple or red is just not worth the trouble
#2 One of the main reasons I liked the game is because we basically had access to an infinite amount of cards from Base, Jungle, and Fossil