In addition to the saddle concern, there's the slurry of road grit being flung into your headset and BB, and the stripe up your back. Sounds like you do have fendered bikes, so I'm sort of confused about why you wouldn't just ride one of them when it's raining.

That being said, sometimes you get caught in it. My least rain-friendly bike seems to be a rain magnet. Even when the hourly forecast is clear, I seem to get wet on this bike more than on others.