Is it imperative to keep the leading arm straight throughout the entire back swing then? My answer is no. It sure makes things more simple and requires less compensations though. Many a good player, even tour players have had their’s bent at the top of the swing. Yet, they do return to impact with it straight.
The real question here is why do many players struggle with keeping the leading arm straight? The answer I believe is two fold. 1) They lack flexibility in their core and upper body particularly the lats and delts. 2) Their back arm collapses. Yes the back arm is the real culprit, not the leading arm. The leading arm wants to stay straight and function as a lever. When the back arm pulls in, the front arm goes with it.
If collapsing your leading arm at the top of the swing is a problem for you, I urge you to work on flexibility and focus on the width of your rear arm. Practice swinging out or back, not up!