If you flip a bad card like Copper/Estate, yes you're dealing out a Copper/Curse, but you're also removing one from your opponent's draw deck and he won't see it again until another shuffle. So the net result for your opponent is one less bad card this shuffle and one more bad card next shuffle. On the other hand, that next shuffle is closer because he has one less card to go through. Overall the effect is more hurtful than helpful to your opponent.
For instance, say your opponent has 3 Estates, 6 Coppers on top of his deck and you KC 3 Jesters, he will immediately reshuffle 3 Estates, 12 Coppers and 3 Curses, ouch!
Flipping a good card is perfectly fine since your opponent won't get to play it until a reshuffle and you gain a free copy of a possibly expensive card like Grand Market.
Flipping a Silver is basically the worst result, because you usually don't want it nor do you want your opponent to have it.