Perhaps in your experience which I don't doubt, but not in mine. Like anything in life one size does not fit all and I don't necessarily think that you need exceptional strength for extension.Bent elbows are most important in my technique. It is thanks to them that I have so much control on the ball. The farther the hand from the body the less contol we have. You keep saying what is lost without straightening the elbow and you are saying nothing what is lost when the elbow is straightened. Only for very strong players straightening the elbow is beneficial. For amateur and older players it is main reason of their failure in tennis.