I generally find that the card is helping your opponent in decks without good trashing. A friend of mine is an extremely aggressive player who buys attack cards without fail. Naturally, he loves the Saboteur. In a game recently with King's Court and no trashing, he TR/Sab'ed me three times in the game, and all he hit were Silver, Menageries, and a few villages, all the while getting some of the crap (copper/estates/curses) in my deck out of the way for me. There were far, far better things he could have TR'ed with more benefit to him.
Also, since they're forced to replace a card, they can easily keep taking estates, low cost non-terminals, or as in my case in one game recently, Duchies, to try to end the game early if they're ahead. This is not a common situation, but I've seen it happen a few times.
According to Council Room, it is ranked 6th in Action Cards in Win Rate Without, and 4th worst in Win Rate With.