I suspect that a twilight grand final is a bit of a compromise position for the AFL. The TV networks and sponsors have been pushing for a later start because those games get better ratings and make for a better atmosphere, but I don't think the AFL would commit to a night grand final on the basis that it's harder for parents, there is a perceived slight dip in quality in play, and they are concerned it ruins the flow of the day (pre-game luncheons and the like).
Personally, I reckon twilight (or late afternoon) is the best time to watch football. The bonus for WA is that noon Perth-time is a rubbish time for the biggest game of the year, and mid-arvo is much more sociable if you're getting people together to watch the game.