The umbrella does have some utility around camp; it protects the open end of a tarp, and provides rain gear for those middle of the night calls of nature, when your poncho tarp is otherwise engaged. Where we live (NC/TN), an umbrella is surprisingly useful on the trail, too, allowing cooler hiking on those warm, wet, muggy days.
Even when using a Tarptent, I have been able to rig my poncho tarp as an awning, providing a nice covered space for cooking and hanging out, and keeping our packs out of the rain overnight. (See? Dual use .) Not sure how it would rig with the Rainbow, but it's easy with the Rainshadow.
You are correct that the hard shell adds warmth. I remember hiking over Big Hump last March in sunny 35-F weather with heavy sustained winds. My usual base layer and wind shirt weren't enough, even with a good hat and gloves, but adding my Precip jacket made all the difference. (Not much use for an umbrella here….)
My choices usually end up like this:
Mid-summer: umbrella only, use tarptent or larger tarp.
Spring and fall: poncho tarp, bivy, rain pants, maybe an umbrella
Shoulder seasons: add the rain shell, take a larger standard tarp, only bring the umbrella if the forecast calls for lots of rain
Winter: pretty much the same.
Of course, all of this is highly individual, and dependent on your own skills and experience.
