I hit my forearm with a bent arm because I feel more comfortable with the ball closer to my body than farther away. Plus, as Lee said, you can't make an adjustment at the last second with your arm if your arm is already straight.
I've tried hitting with a bent arm and it just doesn't feel right.
I'd say it is harder to be consistent with a bent arm. The ease of adjustment can be bad for consistency
I've tried hitting with a bent arm and it just doesn't feel right.
In theory you should be using both, if a flat serve is coming your way and you have your arm fully extended 9/10 you're going to miss it, but if you have your arm bent your using your bodies weight so you can hit more steadier, this also applies to groundstrokes
The straight arm is not forced, rather it is a result of successfully using your body + legs to drag the arm + racket to the ball. If you are holding your arm straight, you're probably weakening yourself.
BTW did novak become more straight arm the last years?
It is still bent but the angle of the arm seems to be more open now. not sure if my observation is correct though.
2009: http://www.youtube.com/watch?v=_TZ2zyfYtLU
2013: http://www.youtube.com/watch?v=PLRLP8g7-zY