I'm more irked by the fact that rain disappears (or appears) when you load up the game, as has been witnessed by other members here. I'm going through my 4th B-Spec 24H at Le Mans, I had to save at 45 minutes or so into it, heavy rain falling at the time of saving, and 100% track moisture.
When I resumed, the skies were clear and bright sun shining on the track, which was still at 100% moisture but drying quickly since now it was a beautiful day! Oh and when the percentage dropped below 90% the AI switched to those elusive
Sport: Super-Softs, which apparently can take some severe punishment before depleting a single line of pixels on the HUD

And the grip they're getting isn't that much less compared to the racing tires I'm packing.
Used to be weather would save itself on our save files when we suspended, the menu screen where the game loaded up the grid would always show crazy weather and track moisture levels true, but at least when we resumed on track, the weather would be as it was when we left off.