I think an umbrella comes into it's own on drizzly days and easier trails---- those where one or no trekking poles are manageable. Walking in light rain often leaves you with the choice of getting wet from precip or sweat and the umbrella is certainly the ultimate ventilated rain gear. A windshirt is the perfect marriage.
Of course the choice concerns hauling yet more weight. I tend to use a poncho or cape which can be as light as the more Spartan rain shells. A sub 10oz umbrella plus a 7oz poncho come to a fraction of an ounce more than my heavy weather rain shell. That combination gives all the multiple uses of emergency shelter, pack cover, cook shelter, latrine roof, tarp shelter "door" and sunshade.
I often hike in cool drizzly rain with high humidity. "Perfect" weather is a myth, spoken of in whispers in order to avoid offending the sun god and causing him to veil his face in gray wool floating in the sky :)
Sunny weather does happen, but the idea of planning on it would be cause for involuntary commitment in a psychiatric facility. Local meteorologists are held in the same regard as politicians, used car salesmen, bankers and the like.