I thought for career mode, it would be like a map where you can see weather. Tracks would have weather randomly. So no race will ever be in the same day or same condition every time you race.
I figured time progression would be in just endurance races, like it ended up being implemented, then again if the game makes us run for 9 hours around Tsukuba you would expect the sun to go from one end to the other... might actually give us a sense of progress throughout the race. Not to mention there was a wet Tsukuba version in GT4, weather could play a part too, anything to shake it up a bit.
When I did the Suzuka 1000kms it seemed like the game was simulating time progression with the weather effect, after lap 80 or so the sky kept getting more and more overcast. I was half expecting rain for the last 20 laps or so, but instead it was just clouds blotting out the sun.
Weather effects would be great on all tracks but that would involve modeling a dry line on every track when the water lets up.