For the "we were talking about when it was a gainer" part, I just fundamentally think you're not even considering the possibility that multiple people trash down, create an engine, and then add jester as payload to that engine to feed off their multiple opponents engines. This is really not that unfathomable and it feels like you're only describing some Jester-BM game without trashing.
Huh? If a card is good it should also be good if everybody has it in his deck.
I keep seeing this sentiment and it's simply not necessarily true. There are two assumptions being made which are false:
1) A card is good only if it comes into play in mirroring strategies.
2) A card is only influential in a game if it is bought.
Like Awaclus loves to say, Dominion is a reactive game, not a solitaire. I'll do a hopefully helpful proof of concept example. Nitpicks about how these strategies are suboptimal aren't going to be relevant.
Suppose there are two competing strategies in a 2-player game:
A) A Scrying Pool engine populated with cantrips, and
B) Council Room BM.
Furthermore, suppose Diplomat and Fool's Gold are on the board.
If player 1 goes for strategy B, player 2 will follow. Diplomat is useless.
For Fool's Gold, it's (hypothetically) calculated that Fool's Gold is only worth it if you can win the split 7-3. If you win the split 6-4 because your opponent delayed contesting FG's to instead buy more normal coins, he will win. Since your opponent can always move later and contest the split 6-4, neither player decided to go fools gold and the game is a standard, boring, Council Room BM game.
If player 1 goes for strategy A, and player 2 goes for normal council room BM, player 1 will eventually create an engine that can end on its own terms and win.
BUT if player 1 goes for strategy A, player 2 can opt to
1) Buy Diplomats and use them as +actions to play two Council Rooms a turn, and
2) Go Fool's Gold, as contesting the Fool's Gold in a Scrying Pool engine would be ruinous so he can easily get the requisite 7.
And in doing so would get Double Province turns and rush to end the game too quickly for player 1.
The default equilibrium of this kingdom is both players playing Council Room BM. But Fool's Gold and Diplomat are still "Strong" cards that are relevant to the gameplay of the kingdom, despite not being bought and being bad to buy in a mirror match.