I live in the Portland, OR area and have an Elinchrom Ranger which is quite weather sealed and would handle a light drizzle without problem.
However, I don't normally use lights outdoors when it's actually raining. Raindrops act like thousands of little reflectors and become very visible when you fire a strobe at them. The strobe also freezes their motion, making them more visible. Unless you are looking to accentuate the rain, you're probably better off shooting natural light.
A cloudy sky can make a wonderful softbox.
You make a really good point. I definitely think there's times I might want to be able to have the visible rain, but I'm also thinking there might be a situation where I might not be shooting directly in the rain, but the lights might be exposed to the elements in transit.
Anyways, it sounds like with a little light rain wouldn't be too bad, and if something were to happen I'll be really glad I have insurance for all my gear!