I have one of the original Golite umbrellas. It is 9.2oz/260g.
The concept is just like any other UL gear-- maximum performance at minimum weight. The Golite I have follows the recommendations of Ray Jardine for an umbrella with a simple design and low weight. It has a solid main shaft, which is strong, but it is always 25" long. I am not familiar with the current GoLite models.
The GoLite Chrome Dome model can provide sun and rain protection. The one I'm using is a light khaki color, so it can work for sun too.
BirdiePal is another brand used by hikers. Note that Prolitegear.com has several models: http://www.prolitegear.com/birdiepal_swing_liteflex.html
I found a ShedRain model that is very light and compact, but also more mechanically complex and more fragile. I found it in a thrift store for a few dollars. 6.9oz/195g, 11 inches long when stowed and the canopy is 38" across-- the same as the GoLite.
It is hard to shop for light umbrellas on the web as they rarely list the weights.
Umbrellas can provide rain protection with maximum breathability. If you use trekking poles, you would have to stash one while using the umbrella. You can use them while cooking and a partial beak for the front of a tarp tent. I don't use mine much as I like two trekking poles and the umbrella weighs about what my rain shell does. I would chalk umbrella use up to personal preference with emphasis towards use in desert conditions for shade and the incidental benefit of rain protection. They do make for more weight and will fail in high winds, which is really where you want good rain protection.