After two
Camino Frances trips, I settled on wearing BOTH a rain parka with pit zips AND a poncho. The choice is not IF I will get wet. I doubt that between perspiration and precipitation that is even possible.
My choice was made based on preventing hypothermia and respiratory problems. I walked both years (2013 & 2014) starting the last week of April and through the end of May.
If I wore only the parka, OR the poncho, I got wet from both rain and sweat. However, if I wear BOTH garments at the same time with the pit zips wide open, my torso (chest) remains toasty warm, albeit it thoroughly soaked from sweat. There have been days when I have had to literally wring sweat out from my fleece underlayer(s).
However, the rain does not add measurably to my wetness. The double layer also helps retain heat. Both pit zips assist my venting excess heat. The good news is that polar fleece insulates almost as well wet as it does when dry.
In terms of preventing hypothermia, warm and wet is okay, if uncomfortable. But, being cold and wet lowers your core temperature and is very bad, especially if there is a breeze or wind.
You can get hypothermic at temperatures lower than 18 - 19 degrees Celsius. It depends on the individual and the specific weather where you are at that time. If you are wet and remain at a cool temperature, it does not need to be in single digits for your core temperature to drop.
So, I tossed the weight argument after I had health problems the first Camino. On the second Camino I packed the parka, poncho, TWO fleece layers (one form-fitting 100 level fleece 1/4 zip pullover, and the second a zip jacket, 200 level fleece).
I also bring fingerless microfiber gloves to protect my hands while using poles, against sunburn and to help retain warmth in the exposed extremities. I use a pair of tan microfiber fishing gloves, sans fingers. Lightweight bicycling gloves also work well.
Finally, I top it all off with a Buff, tucked to be worn as a watch cap, under my ballcap with the brim that keeps rain off my eyeglasses. I hate having water drops on my eyeglasses...
It all actually works, for me at least. In my opinion, would rather carry an extra kilo of proper kit than be sidelined with illness, or have my Camino ended by same. It happened to a friend of mine this year.
I hope this helps...