Bishop is a generally good opener.
Bishop is generally TERRIBLE as an opener (there are exceptions), yet people do it ALL THE TIME.
Can you explain this more?
the main downside here is that bishop offers your opponent an option to trash as well. they can then focus on building their engine and playing other action cards while still thinning out their deck just as fast as you. this boost is often enough to offset the marginal difference in VP chips in the early game.
Yeah, so this is basically correct, but I would like to expand on this more.
So if you look at trashing an estate with it, and your opponent does the same, you come out with 1 more money and 2 more points, whereas he has a, let's say silver in his deck (the opportunity cost). This is a pretty decent trade.
You trash a copper and he does the same. You end up with 1 more coin, and 1 more point. Not so worth it. Now overall, we look at BM-bishop opening with bishop being best, and it beats other options... but only very slightly.
More important, you are going to have a 5 card hand, one of which is bishop, which gives you 4 more cards to trash from. Your opponent has 5 cards to trash from. So he has extra chances to trash what he wants than you do, and he has extra cards to do something with after he trashes, and you've used a terminal action.
Now, bishop is by no means a bad card overall, for a few reasons. So why? Well, basically it is at it's best in the golden deck (this is of course a special case big exception to my generality), or in an engine, later on. (Bishop-BM is like, I think it's even worse than chancellor? It's really bad, almost never optimal - very many things let your opponent win the province split, and you have a very hard time hanging in the duchies, or getting 12 points from chips in time...).
Why is it better later? Well, there's a couple really big reasons. First, it's less likely that your opponent can just trash something without hurting their next hand in a significant way, which takes a way a huge portion of the drawback. Moreover, you are often reaping LOTS of points in such an engine (trashing 1-2 golds and re-buying them every turn is really good, and a really good way to crush a BM player to death, for instances), and perhaps most important, you just have more cards in your hand to be able to trash exactly what you want.