From a user point of view, I would consider it a bug it the variable browser was not in sync when octave is idle.
Just a simple example: you have a global variable, that is modified by some graphics callback. Not keeping track of the change would mean that, after triggering the callback, the user would have to press the Enter key in the octave console to see the variable updated (or pressing some hypothetical "Refresh" button in the variable browser). This doesn't sound very user-friendly.
Michael.