I agree with russblau. Rain often moves quickly and can start or stop at any time. When I see a rain report after I've been in the rain for half an hour, it is just really dumb. I'd rather just get rid of any weather report and isn't going to be stationary just like we don't report moving vehicles. For the same reason... you don't know where it will be at any given time. You can assume that it will be in the area for awhile, but there isn't much guarantee. Also, if you can't figure out that it's raining, you shouldn't be driving. In most cases, it will start sprinkling first, which isn't a danger. That gives you plenty of time to prepare for possible heavy rain. On the more rare occasions where there is a "wall" of heavy rain where you go from no rain to pouring instantly, you can very easily see that wall as you come up on it and can slow down (if you're smart) before hitting it. Really heavy rain is where the real danger is. I know that in some states like NC, even a light rain means wrecks, but that's just bad driving and not really because a light rain is dangerous. Anyhow, marking heavy rain might be fine except that heavy rain typically doesn't last all that long. Clouds can only hold so much water and if it all comes down quickly, then the water will run out quickly. Exceptions of course being hurricanes and typhoons that continually bring more water into the sky.
Sun glare isn't a bad idea for locations where it happens unexpectedly. Marking the sun just because you're heading west at sunset on a flat road without any trees isn't good. But if you're on a road that is protected from the sun for almost its entire length and then suddenly it isn't, or it passes by a lake or river that greatly intensifies the glare, then having a warning at that point isn't a bad idea. Most of these places do have posted signs warning of that, but an audible warning can also be helpful.
Michigan - Northern LP.
Waze running on Samsung Galaxy S3.