I knew that someone would be working on this. In fact my monies on more than one person working on it for their mods/general release. Of course, the biggest issue is how it will affect the game's engine (all those particle effects unless they are doing the rain differently) and how good it looks. The reason so many game companies steer clear of rain in first person games is it can affect the game's performance and doesn't necessarily look realistic (no wet weather reflections, rain not impacting on water, raining under structures when it shouldn't, etc.).
I look forward to seeing how Minmay handles it in his mod.
EDIT: I should clarify that many game companies might want to add rain, but to make it look good can take a lot of time and effort better spent elsewhere.