Is definitely the best option for technical clothes for cold weather. Try and avoid the blends and look for as close to 100% as possible.
I'd say that if you want your merino wool to last, it's imperative to look for blends. Pure merino wool is extremely fragile. Whether that's bad for the environment or not, I don't know.
Blends need to be washed way more often. If you don't overwash your pure wool it will last well.
Source my clothes.
Except that this particular discussion is about using wool in performance athletic gear.
I know I use wool for cross country skiing and cycling in cooler months. These clothes last several uses. When I try the same with plastic blends they need washing after one use.
EDIT. Several uses closely spaced and the items are aired out between use. If you sweat all over some wool then leave it in ball for a week it will probably rot.
I don't find this to the the case at all. Most of my base layers are blended merino and they need to be washed just as rarely as pure merino. Perhaps not all blends are created equal.
Interesting what percentages are we talking? Something like 30% is enough to ruin the garment for me. Maybe I am just very smelly!
So I looked at my clothing, and most of it seems to be 87% merino wool, which is less plastic than I expected, however in my experience that 13% makes a huge difference in terms of durability.