I'd say it's a comfort level for you: how well do you know the road? do you know it well enough to feel you can ride it in the rain?.....also do you have a headlight et al for dark/poor riding conditions? If you don't feel comfortable riding in those conditions: don't. The bike won't mind (and less cleanup afterwards! lol)
Life should NOT be a journey to the grave with the intention of arriving safely in an attractive and well preserved body, but rather to skid in sideways, champagne in one hand, strawberries in the other, body thoroughly used up, totally worn out and screaming: "Yeah Baby! What a Ride!"