It might be because the umbrella also can provide some shade, something that is rare on some parts of the VF (from what I have read). I plan on carrying an umbrella for my Canterbury through France section of the VF this spring. I will probably also bring my ultralight poncho (actually a DIY silnylon Parcho) as well.
A serious umbrella to be used in hicking and Caminos has to be strong and wide enough to protect you - at least the whole upper part of your body and backpack - in a decent way and to face strong winds as well.
Personally I don't like the umbrella, because when you use it you can't use the poles, and when you tie it to the backpack it's quite cumbersome.
My warm suggestion for you is to try the umbrella on a day trip and then decide if it is worth taking it with you for a month or more.
I walked from Canterbury to Rome between June and August and carried a EuroSchirm Swing Hands Free Trekking Umbrella. It was a great aid providing shade when walking in temperatures over 35 deg. It was light in weight and the clips provided by EuroSchirm made it truly handsfree.
how do you cope with the weight and 'stickiness' of the wool when it gets soaked (on top of the its normal weight)? that was one of the reasons I opted not to use a woolen cloak for my medieval camino. or you are referring to a shorter cape that just covers the head and shoulders? or just the shoulders?
Trouser legs do, but often except in the seriously heaviest rains (think those serious Galician rainstorms if you've ever been under one), the back of my t-shirt and the seat of those trousers will stay dry.