I like to run proxy cards/decks as I don't play with any official cars that cost's over $15, and I use many of my favorite cards in multiple decks, so buying multiple copies isn't feasible for me
A single destroy spell can become a board wipe that empties the battlefield
A five mana planeswalker, with five starting loyalty, that has a plus, a minus and an ult
Spreading out your threats is always a good idea, and if someone plays an enchantment or creature only boardwipe, due to having no artifacts to fall back on, youre going to get behind on other players
Since it enables so many things, it is likely to play a key role in many standard decks throughout its life time, earning it a spot in the A tier
After all, you cant play that super-awesome, giant creature if you dont have the mana for it