I sometimes use the root helmet in swamp for the poison resistance, but, as long as (as I pointed out in another post) you don't shower in the green mist of a blob, iron provides plenty of resistance to poison, so it usually goes on the shelf pretty quickly.

I've been trying out the harnesk lately. It is nice to wear around the house in the plains to resist the occasional dive bombing deathsquito before breakfast. It also helps against ticks, especially the starred ones. The non-starred ones are not normally a problem if you have an AOI weapon, or are quick at rolling, but the 2* ticks work fast, especially in lighter armor. I don't find any use for it otherwise, and I'm only using it now because I'm using all light armor. With heavier armor, I don't think the tradeoff is worth it.

Fenris, on the other hand, is awesome. It's good all the way through mistlands, especially if you have a high move and jump skill. It's alot of fun combined with a quick weapon like a knife.

Going the light track, I skipped iron (of course) and just went straight to Fenris armor from troll. You have to be careful of the cultists in troll armor or you turn into a pile of ash, but it's well worth it.