I don't consider myself someone who babies her bags, and I'm not neurotic about keeping things new, but I do avoid using any of my expensive handbags in rain when I can get away with using a casual nylon tote. Part of the reason for this is that I fear having any leather dye bleed while wet and cause a stain on lighter clothing (this happened to someone here, if I recall correctly, with a cobalt bag (SS 2009, I think). The other reason is that I figure that I don't need to carry a designer bag on every single occasion, and so I give my BVs and other high-end bags a break on bad weather days.
If I need to dress for a certain occasion and want to use one of my BVs, I use an umbrella, for sure--especially if it's my karung Stretch Knot that was a tPF SO. It's irreplaceable.