Whether you're running Amethyst/Steel control or Amethyst/Sapphire"Blurple", these cards are non-negotiable additions to your deck and some of them are flying under the radar while everyone's distracted by the other flashier reveals
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
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
There is also a potentially quite effective synergy with Wrath of God
If youre just here to read what the best is, there you go
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