Hitting to the the middle is a useful defensive tactic, because it limits your opponent’s attacking angles.
The downside of hitting to the corners
Hitting to the corners gives your opponent better angles for his shot placement.
Suppose your opponent is playing a drop shot. From a corner, he has good angles to play straight or cross-court. You must bias your base position slightly to cover the straight shot (because the shuttlecock takes less time to get there).
Because your position is biased towards covering the straight shot, the cross-court angle becomes more effective for your opponent.
Compare this to when your opponent is playing a drop shot from the middle. Here, you can stand exactly over the middle line, and both angles are equally easy to cover.
Movement pressure is usually more important
Most of the time, the benefit of applying movement pressure greatly outweighs the disadvantage of giving your opponent better angles for his shots.
If you always play to the middle, you will have few opportunities to win the rally. This is because you are not placing your opponent under enough movement pressure.
Your opponent, however, will continue to pressure you, by hitting to the corners. So you’re under movement pressure, and he’s not. No prizes for guessing who will win that game!
When defending, you want to limit your opponent’s angles
Playing to the middle is a wise decision when you feel the rally has turned against you. For example, a high lift to the middle neutralises most of your opponent’s advantages, and allows you to recover. Because the shuttlecock is in the middle, your opponent’s shot angles are less dangerous.
Playing your shots to the middle is a purely defensive approach (apart from smashes and net kills). It is rarely an effective way to gain an advantage.
An alternative approach is to counter-attack by hitting to the corners. In this situation, you are trying to turn the tables on your opponent by putting him under movement pressure.
As a general guideline, you should continue to hit to the corners even when your opponent has a slight advantage. When your opponent has a strong advantage, however, you should hit to the middle in an attempt to limit his shot angles.
The best purely-defensive shot is a high lift or clear to the middle, provided that you can make it land near the back line.
High shots should always go to the middle
High serves should always be played to the middle.
If you play a high serve to a corner, you fail to create any movement pressure: because the serve is high, your opponent has plenty of time to move into the corner and get behind the shuttlecock.
Playing a high serve to a corner does give your opponent the best angles of attack, however. So you’ve given him all of the benefits of a corner (angles of attack), while imposing none of the disadvantages (movement pressure).
The same logic applies to very high lifts and clears (defensive lifts and clears). If you play a very high lift or clear, your opponent is under hardly any movement pressure and can practically walk to the shuttlecock! Again, you’ve given away the best angles of attack.
So whenever you play a high serve, or a very high lift or clear, play it to the middle.