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There are three reasons why the straight drop rather than the cross court should be your percentage shot.
Firstly it is a shorter distance for the ball to travel so your opponent has less time to get it!
Secondly the ball stays closer to the side-wall for longer giving your opponent giving your opponent an added problem!
Finally it is easier to cover both sides of the court when you play straight. One step back and you can cover a straight or crosscourt reply. After a cross drop it is difficult to cover a straight reply so it had better be good!
That is NOT to say never play crosscourt drops. Use them as a variation, or in desperation when that is all you can do to keep the ball in play.