If you're fighting opponents that are using early-game Bishops to trash their Estates and Coppers, you want to use the free trashing they're giving you to improve your deck to the point where you can make up for their VP-chip lead with extra Provinces or Colonies. Usually that means you're building an engine. With Library on the board, there's no need.
Library is a powerful Militia counter because you can discard your two worst cards and draw better ones to replace them. Similarly, an opponent's Bishop lowers your handsize by allowing you to trash a bad card from your hand, which is even better for you. Trashing Estates to your opponent's Bishop is usually a no-brainer, but normally when you trash a Copper you're sacrificing $1 of coin this turn. Not so if you have a Library in hand. Odds are you'll draw something better in its place.
It's hard to overemphasize the tempo boost that this gives you. If you're playing against an opponent with Bishops, I recommend about 3 Libraries. You'll be buying Gold and Platinums with incredible speed and zoom to a quick victory while your opponents struggle to build up their decks. As long as you stick to buying only the most expensive Victory card on the board, you should be able to grab 8 of them before stalling out.
