Sometimes it's a good tactic to go after an opponent's strength. After all, his game is probably based on getting that shot into play, and so you are probably going to have to face it - so rather than have the opponent choose when he'll use it, why don't you pick choose those times?
For example, suppose your opponent has a very nice forehand loop. He's going to use the shot; there's no stopping that. You could play into his backhand, but then he could step around to use the forehand, and he gets to choose which shot he wants to do it off of. So why not simply attack his forehand side yourself, and force him to use his strength off a difficult ball, and then come right back to his backhand side, where he now has to play his weaker shot while moving?
Or suppose your opponent is a very good blocker. You keep getting stuck in rallies where he's quick-blocking the ball around the table, rushing you and forcing you into mistakes. Since he's going to block anyway, why not throw a slow, deep, spinny loop at him? That's often the most difficult ball for a blocker to quick-block - he has no speed to play off, it's deep so he can't really rush you, and the spin makes it tricky to block. And so rather than getting quick-blocked all over the table by his blocking strength, you'll get a weaker block that you can really attack. You've turned your opponent's strength into a weakness.
If a player has a good loop against deep serves to the backhand - whether forehand or backhand - you might be able to turn this into a weakness. If you serve very short to his forehand, he might have to stay closer to the table when receiving then he'd like - and now he gets jammed when you do give him that deep serve to the backhand.
Similarly, you can find ways to negate an opponent's strength and turn it into a weakness. When you do so, it's a double-whammy - you've both taken away his strength AND found a weakness!