I no longer feel like I understand Bishop at all. In theory, you don't want it early, because you'll just be helping your opponent thin his deck. And you don't want it mid-game because it'll kill your economy. And you don't want it late because trashing Provinces costs you 2VP AND an action point AND a card slot in your immediate hand just to get rid of a dead card. And yet people who just buy Bishops and trash everything in sight often beat me. So what exactly is the proper use of this card? Assume no obvious enablers like Hoard or Chapel.
Early-game, the VP advantage of Bishop vs. no Bishop is extremely strong, often enough to lose the Province split and win (and after the three Estates, you're giving your opponent the choice of -$1 or no trash). The exception is Colony games, where you're not going to get a 20VP edge, +$1 for two cards is abhorrently weak, and you can't reasonably trash Colonies for VP. Early Bishop in Colony usually loses the Colony split and thus the game.
Mid-late game it is indeed bad for your economy in BM, but the VP edge can be huge, especially if you end up with excess Gold.
Endgame isn't the ideal time to pick one up, but if you've been efficient, you can trash Provinces at -1 per Province to keep your deck humming; or you can trash things like Gold to pick up big-VP turns without a Province buy.