I'm kind of curious where the 'rage is an emotion' idea came from, especially as there's an implied 'only' in there (rage is
only an emotion) which seems to be the basis of the 'rage stops when you go unconscious' argument.
Rage as a class ability is more than just an emotion, though - it's something with a defined physical effect. If it were emotion only, then any class should be able to rage. After all, clerics and fighters and rogues can get angry too, can't they? But since rage is only available to the barbarian (and various prestige classes), the 'just an emotion' argument falls apart. There's got to be something more to produce those physical effects.
As someone else has pointed out, the physical effects of adrenaline seem very similar to those of rage. The effects of adrenaline do not instantaneously 'shut off' when one goes unconscious. Of course, this is bringing real-world logic into a fantasy game, so it shouldn't be seen as a definitive argument - just one that provides a justification for what is really quite clear in the rules.
It is definitely a conscious decision to enter rage, but the language of the rules implies that it is equally a conscious decision to end it early - in other words, the barbarian has to 'get control of himself' or will himself out of the rage. Obviously if he is unconscious, he cannot do this, and the physical effects of the rage will continue until it burns itself out (ends the duration normally).
To paraphrase: the benefit of this ruling is that it makes more sense, and it does not instantly kill barbarians above level 5 who go below 0 hit points.
J