I've been caught out in a downpour with my Bals. They've all dried ok. Ive even carried my Suede Bal on a showery day where it didn't stop showering at all. I tried very hard to cover it but some parts still got splattered. I got home and it was all ok. I waterproofed it as soon as I got it, which was over a year ago.
I wear my Bals when there's light showers, but when it's truly wet out, I carry my Goyard.
With the weather being so unpredictable this year, I've been caught in the rain before with my Bals, particularly my A4 Papier which got absolutely soaked. Thankfully, there was no damage at all, but I wouldn't make it a habit.
I usually try to avoid bringing any of my Bals when I know it's going to rain/it's raining. I rotate my bags frequently, and tend to use one of my "rain bags" for rainy days (Prada nylon, Longchamp nylon, LV canvas, Furla Candy, RL canvas). None of my Bals are treated/water-proofed so that also plays a part in my decision.
Last Friday night I got caught in a huge downpour, as in buckets, of rain carrying one of my Velos. We literally had to go from one end of the outdoor mall to our car on the other end of the mall. I freaked out thinking my poor bag was going to see some damage. It dried and was absolutely perfect! I have carried Bals for years and knew they are usually fine, but I think I was more worried about this one because it's one I purchased recently and the thought of my new baby having damage was scary. Like others have said, unless it's suede, I don't look at the weather report in deciding what Bag to carry.