for myself I have to say that I use a different defense style in doubles then in singles.
In doubles I (try to) use the thumbgrip to play a smash high back to the enemies baseline - when the situation is good and no chance to play a short net shot return.
In singles I don't use the thumbgrip because I need all the lenth of the racket and don't have to play it back high and can also return a smash as a short net shot and thats easier with other grips then the thumbgrip.
Do you confirm with me? Is this correct or how do you defense in single/doubles?
In double, not only using more of thumbgrip, I hold the racket higher up the grip. I find it much easier to response to fast play.
At back court, when I have time, I hold it normal as in single, for smashing.
The problem is the opponent's tricky half lift defense, making me move forward, not able to smash, changing grip, changing the length of racket.... drive the birdy into the net more often than not....
Haha there is no 'right' or 'wrong' way to hold a racket. It all depends on the situation.
If i were your opponent, knowing this i'd keep attacking to your forehand side when im playing doubles with you because you always hold the backhand grip, and similarly if i play singles i'd attack your body because you are prepared to reach out the the corners and not prepared for back hand body shots.
So my advice is that you need to know what are your opponent habits, or in other words, you have to anticipate your opponent's attack. And usually in doubles most attacks are very nice to defend using thumb grip(backhand grip). And if it's working for you, keep using it!
As for singles, you really have to evaluate if its the right kind of grip depending on your opponent.. You could even try standing a little to the left, or the right, depending on where are your opponent's deadliest smashing position and habits!
So keep exploring with different different positions alright and good luck!