No one wants to have their The Ur-Dragon taken down by a 1/1 cast by for a single
I spoke to Dave Chalker, an extraordinary game designer with deep expertise in both chocolate and peanut butter (I think D&D is the chocolate I forgot to ask Dave)
As such, if Vivi has the power for it, it's better to use Vivi's effect for mana rather than from lands or mana rocks to cast spells on your turn
It could pop up again sometimehopefully this time in a set where it isnt overshadowed by some more controversial mechanics
You get to draw a card each time you play a decently-sized creature, and they have trample to boot
With two cheap commanders in the Command Zone, you get to use Fierce Guardianship and Deflecting Swat for free, likely much sooner than your opponents, and much like Rograkh, you can offer up Silas to any cards that require a sacrifice to reap those rewards