More mana means bigger plays, and bigger plays usually decide multiplayer games
When Nethroi mutates, you return any number of creature card(s) with total power 10 or less from your graveyard to the battlefield
With Zinnia, Valleys Voice, you can give all your creature spells offspring for two generic mana, letting you make a 1/1 token copy of each creature spell you cast
In my experience, MKM teaches tight sequencing and rewards thoughtful play while still providing Standard upgrades you will use for months
You might have a few dozen of them, but theyre all just 1/1 and 2/2 creatures at the end of the day
Having said this, we can already see a scenario where detrimental counters (such a finality) can be mitigated in impact through Tiduss first ability