I think that there should be a realtime weather option in minecraft that changes the weather in minecraft to the same as it is outside. So it would be raining on Earth and raining on Minecraft. Cloudy here, cloudy there (teehee it rhymes!). The reason it should be an option in the options/settings area is because some people might want to see some sunshine on a rainy day.
For SSP though, sure. As long as it is optional.