How would it benefit the server: I think there should be a every once in a while time on the map where it starts to rain, or have a thunderstorm, I think it would look realistic and give some taste in RP, I don't believe it would benefit people but I also believe it would not negatively affect people. I think that this could make ZARP look awesome!
Potential Issues/Exploits: I don't think any issues would occur.
Additional notes: Thanks for checking out my suggestion! have A great day.