I have a hiking umbrella, but have never been able to reconcile with with trekking poles, with the poles winning out every time. It would be interesting to have one that incorporates a fixed length trekking pole, or has some sort of sleeve/shaft that could cap a pole(sans basket).
I could see a flat river valley hike using and umbrella and that is usually at lower elevation with lots of cold humid air, which is just where you need the breathability. If there is a light drizzle, you have the choice of sweat-wet or rain-wet. That goes for your head as well as your torso. Umbrellas can make for some shelter for the cook in camp, strolls to the latrine, help plug the weather end of a tarp, or give your head end a little relief when camping a bivi sack. The "chrome" version adds sun protection.
I can't see escaping the need for some backup rain shell or poncho. Something like a DriDucks and umbrella might make a good pairing. I would be wearing rain pants regardless.