Bishop silver gives you a good chance of hitting 5 first, especially if your opponent does take the advantage to trash. With a strong 5 card like minion I'd say it's worth opening with.
Opening Bishop/Silver is worse for hitting 5 than opening Copper/Silver.
Thank you, finally another voice of reason. Not to mention that you have almost no chance of getting TWO fives turn 3 and 4, which what, maybe 10-15% of games if you open silver/silver? To get two fives with bishop-silver, you need to draw them together along with one estate and two copper, have a hand of five copper, and have the two estates in your last two cards. That's got to be sub-percent level.
What 5's are you really going to want to pick up 2 of turn 3 and 4, if you also wanted to open a Bishop?
Plus, that's no different than say... remake or any other trasher. ALL of those cards give up slight early buying power in order to have superior buying power later. Here, while the "trash" benefit might be equal, the pay off benefit between the two players isn't. If your opponent has SCCCC when you play your bishop, sure they can trash a copper, but then they give up gold. So, it's not as simple as "You won't get two 5's"
I DON'T want to open bishop. I don't even care if I pick it up till turn 6 in many cases. And you CANNOT compare bishop to other trashers, because it also lets your opponent trash. Opening with it DOES NOT give you better buying power (than your opponent) later. Steward lets you trash 2 cards, for instance, which is completely different than trashing 1 estate, and letting your opponent trash an estate too (and 2 VP). For the first one I'm willing to take a hit in the number of 5's I hit, for the second, I'm not. Opening bishop gives you VP advantage, NOT deck advantage.
5's where I would want 2 of, turns 3 and 4: Wharf, Minion, Stables, Laboratory, Hunting party. I'm sure there are others, but those come to mind. Particularly with something like wharf, the extra buys will make it easy to buy a bishop later.
Sometimes I'll even just skip the bishop for a good 4: say ironworks is on the board, I'd rather start with ironworks for cheap engine pieces, and then use the ironworks later to gain a bishop, then bishop fuel. Or start caravan, trying to hit 6 for hoard, buying bishops later.
I just think getting your engine going is often more important than the few piddly points you gain from a bishop opening.
I'm not saying its always clear cut. To be honest, a case like minion - with no other trashing, and say a village as well, and no extra buys: its a close call. I just still think, even in that case, that opening silver/silver is ever so slightly better.
Mostly I think people just go on autopilot and open bishop, and its wrong much more often than people think.