'Because the RAF says so' is not an acceptable answer.
To give credit where it's due, the RAF did operate the Chipmunk professionally, continuously, for about half a century, God knows how many hours and has a wealth of operating experience that should carry some weight. In my experience of RAF Chippy operation, half flap
was used, the carb heat was
not wirelocked hot and brake
was applied on landing as described.
If you land with brakes off, you give yourself all the options; you have FULL rudder travel available should you need it. If you ground loop and even full rudder won't hold it, gently pulling back on the brake lever will apply the appropriate brake
By then you've left it too late. The two notches of brake may restrict rudder travel slightly, but when
on the ground, increases the yawing moment that you can apply to the aircraft and hence increases the control authority. The application of differential braking on all Chipmunks that I have flown has been progressive and not abrupt.