In the rear court, you don’t want your racket ready too high, otherwise you won’t get your preparation quite right with your overhead shots. The rear court is ALL about staying relaxed and opening your chest out. So if you have your arm up from the beginning then you won’t be able to open out as much and you’ll lose power, accuracy and control!
You also don’t want your racket up very high when you are defending, in both singles and doubles! The only time you would have it at waist height or above is if your opponent was taking their overhead shot late and you think you have an opportunity to counter attack.