I believe someone noted previously that the reasoning is when there is no whistle is so the O knows it’s a 40 with no whistle if and when they hear a whistle they know 25. If you blow a whistle every time with no visible clicks there would be confusion as to if it’s a 40 or 25.
Perhaps, I haven't stated my concerns clearly, allow me to try again. I'm not suggesting whistles are necessary at "40 seconds", that event seems clear enough on it's own (a previous play ends inbounds, a pass or kick ends in/or OOB). Thereafter we have provided a consistent alert (Referee's whistle) when the shorter 25 second RFP interval began, which momentarily provided a notice that alerted EVERYONE to a specific reduced time interval.
The current alert, to reaching that reduced time interval, is set by who may be watching the movements of the Umpire and the placement of the ball, which may often conflict with, or be distracted by, Coaching instructions, player discussions or other distractions, that were previously overcome by a specific, and unique audible Referee RFP whistle.
Despite DECADES of availability, only a percentage (far short of universal) of playing fields
(servicing INTERSCHOLASTIC SPORTS) include visible 25 second play clocks, and there is NO assurance that the installation and/or conversion to add 40 second play clock capabilities will happen any faster. As with the adoption of 25 second field play clocks, manual signals being adopted by field officials are suggested to help with recognition, until the availability of visible 40 second clocks become universal, which considering the deployment history of 25 second clocks, may likely take DECADES.
At the NCAA and NFL levels, where the 40 second interval was initiated, the presence of visible play clocks supporting that process is ESSENTIALLY UNIVERSAL.
Presuming there still may be a practical value to INSURING everybody on both teams is (gently) reminded that the ball has been rendered RFP, continuing the ages old practice of a "Referee's whistle" seemed logical and universally consistent (at least until visible play clocks and/or some new technology rendering them unnecessary replaces them).