I know what you mean about the weather and whatnot. I'm also trying to get my Sierra summertime load down as light as possible. And there are definitely times when you KNOW it's not going to rain, even thunderstorms. But you can almost always get some funky bits of rain in the Sierra, so...
I'm going the route of a MLD Sil Pro poncho/tarp. So for 10 oz or so, I've got raingear, packcover, and shelter. I'm going to skip the bivy on a typical trip. If I think it's really going to rain, I'll either take a MLD bivy, or more likely just the Tarptent Contrail and raingear. But for those usual Sierra trips with great weather and maybe a bit of afternoon rain, the poncho/tarp seems like a good plan to me.