Atos' post #12 is precisely the sort of argument that fails miserably. Fischer did indeed play, and win with the KIA. He even played it relatively late in his career.
But against the Sicilian, his favorite variation was the Open Sicilian. You don't see these KIA afficionados recommending the Open Sicilian main lines "because Fischer played them".
When Fischer played the very best in the world as White in the Sicilian (Taimanov, Petrosian, Spassky), he chose the Open Sicilian.
You are right that Fischer played the Open Sicilian, and in fact was probably the greatest expert of his day on the Open Sicilian. So, if he also played the KIA against the Sicilian, it certainly wasn't due to laziness or ignorance of opening theory.
I think the KIA is played a lot among we patzers because it can be a lot of fun and, in most lines, offers a very simple plan: attack like a madman on the kingside and try and get to white's king before he eviscerates you on the queenside.
Hardly sound chess and it doesn't help you get to the next level, but it can be a lot of fun and a way to improve your tactical skills without having too worry too much about opening theory.
In short, it has value as a bridge between novice and learning to play somewhat seriously.