The rules text specifically says, "When a balanced card falls or touches another balanced card, sacrifice Phyrexian Librarian." What happens to the balanced cards after the Phyrexian Librarian leaves play
Not only does he disable all of that companion's card bonuses for the turn, he also constricts card flow and makes twilight if the player replays the cards next turn
The cost of may not sound like much, but any equipment that raises Esper Sentinels power will get the cards flowing your way very fast
It only takes three defense counters to defeat, tying it for the least among the battles, which is also solid, and then the backside is fine, as a 4/4 that can make some extra mana
For instance, the Captain America Groot has grown a lovely floral shield, while the Wolverine Groot has grown leafy sideburns and hair tufts
Making tokens is great with it, putting extra lands onto the battlefield is great with it, and it can sometimes be hard to turn these things into real material gains so sometimes you can get a lot of them at a pretty low rate
