IMO armour sets are pretty much predetermined for certain builds. If you have a mix-up build like a dual wielding battle mage or something like that, you're not going to take full advantage of them. Though these sets can be pretty useful early on because of two reasons- two enchantment slots and skill boost that they provide. But some unique items which are not part of any set also provide skill boost (Amdusias' helm is one fine example) and once you get your hands on elder/epic level armours you can replace those predefined sets. Early in the game I found the Aleroth defenders set to be somewhat useful, even for dual wielding set-up. Another thing I've discovered that sets are meant to be used as a whole, the more you add pieces the more their individual power grows. So either put on the whole set to get most out of them or use none if you have better or equal alternatives. I was using blood echelon for a while but it requires to hold on to the shield so ditched it for a custom set-up. I like the looks of that armour which Captain Luis used to wear but unfortunately didn't find it in elder or better variant.
