As a Triathlete, even when its pouring rain I put on my shoes, or goggles and trudge out the door. (If I'm swimming I'm going to be soaking wet anyway.) But is enough, enough?
I'm in Binghamton right now with the remains from Hurricane Lee. It's pouring rain, the streets are flooded and you look like a drowned rat when you walk from your car to the house. Is there a limit to the amount of rain? Some say yes, some say no...
Sometimes its good to say "yes there is a limit." Sometimes it's not even raining that hard. As an athlete there is ALWAYS some weakness that can be worked on. Do a really good strength or power workout inside. Or do agility drills in the living room. Do isometric strength exercises across the couch, in a chair, or on the floor. If you live with somebody you may look like a complete idiot but that's OK! It'll keep you mentally strong rather than going out in the rain.
On the other hand, if you've been cooped up in the house for days and have neglected to train for a few days, gaining the courage to get out in the rain may be exactly what you need to get back into it. The next workout you do will feel light and easy. You may even have the greatest workout of your life in the rain. Go for it!
So it all depends on the situation. If you've been overtraining or are just mentally tired, try some indoor stuff. If you've been neglecting your workouts, fight the rain!