I have an Osprey Kestrel with the attached (velcro) raincover. I use the rain cover in ALL precipitation. Here is why...
In light mist, the rain cover, ANY rain cover, protects the pack, but not the shoulder harness or the portion of the rucksack against your back. If the rain increases, water will drain off your back and into the unprotected back panel and harness. Your pack will wick water. Your stuff will get wet, or at least damp.
In a steady drizzle or any definite rain or snow, the same thing will occur unless you are wearing a pack poncho or trekking raincoat. This is a poncho made to wear OVER your pack. It covers you, and your pack. The trekking raincoat is a knee-length front opening raincoat with sleeves and a hump back panel that unsnaps to cover a pack.
YES, some might consider it redundant to the rucksack rain cover. However, consider that, as soon as you put on a poncho or trekking raincoat over both you and the pack, your perspiration will condense on the inside of the poncho and run down onto the pack. You cannot prevent this.
So, if you wore the poncho or trekking raincoat without the pack cover, your pack would still wick and absorb condensed perspiration from the inside of your poncho or trekking raincoat. There is no getting away from this. You would run the risk of arriving at your night's lodging with wet stuff inside your rucksack. The items will not likely be soaking wet, just damp and uncomfortable.
No rucksack, except a specially designed "dry bag" can prevent the wicking. These special bags are heavier than the combination rucksack and rain cover you currently have. Most rucksacks are water resistant, NOT waterproof.
On balance, my advice is to use the rain cover whenever there is precipitation in the air, and use the pack poncho or trekking raincoat to provide improved rain cover as needed. There are many brands of rain ponchos and trekking raincoats available. I have experimented with several approaches on my two
Camino Frances pilgrimages (2013, 2014).
In my considered opinion, a pack poncho made from "ny-sil" or "sil-ny" (siliconized nylon) is the optimum solution. These ponchos are lightweight, water resistant, fast to dry and you can find them in bright colors. I recommend a bright color so you contrast with your drab surroundings. The contrast is what makes you visible to others, especially oncoming drivers. Yes, they are more expensive. But, you will see many discarded lesser quality ponchos along the Camino, and not all of them disposed of properly.
I particularly like the Sea-to-Summit offerings (
http://www.seatosummit.com/product/?item=Ultra-Sil®+Nano+Tarp+Poncho&o1=0&o2=0&o3=189-41). My present poncho is light blue. I preferred the brighter lime-green, but it was not available.
Several people have commented to me that the snap fasteners for this poncho are not tight enough. It is true they are nylon plastic and are not as tight as the brass fasteners frequently found on other ponchos. However, I have a nifty fix for this. I obtain 3M reflective tape from my local DIY store. Cut pieces 1" x 2" for the total number of snaps on the poncho. Snap all the fittings together as you would to wear it on the road. Apply one reflective strip over each fastened snap so it forms a "U" over the snap extending about 1" on each side of the snap.
The result is a poncho with seams that will not easily come apart and which now has 8 reflective patches facing front, and another 8 facing to the rear. I recall there are four snaps sets on each side of the poncho. If needed, you can always remove the tape or slice through it on the edge, carefully to open the snap. However, in my experience, once the snaps was fastened and taped shut, I did not need to open it again.
The first time I tried this, I used 1" wide strips of duck tape. This worked until one very rainy, windy day, when the tape tore and a couple of snaps came apart. I now have 3M reflective tape over my snaps and will use it on my next Camino in March, I think.
There are lighter ponchos on the market being made from Cuben fiber fabric. However, these ponchos are very expensive. I submit they are presently not worth the additional cost. But that is a personal, individual choice.
I hope this helps...
) It's so compact and takes up no room so it will be with me in an outside pocket. I have not tried in in high winds but I like your idea about the reflective tape. I'll try that.
i've only seen one person use that umbrella before on the camino, but not against rain, but sun....HOT Mesetta HELP there is no shade type of Sun. 
