I discovered this as well! It led me to try giving companions other useful items, such as spell books and staffs. However, it doesn't appear that they utilize these items. I even tried spell books/staffs with a pure magic based companion. Regardless, she still summons atriarchs and can take an elder dragon down without my help if I let her, so I can't complain.
As for my dear Lydia; I was using her for the majority of the game - even had her a full set of Dragonbone Heavy Armor. I'd engage large groups of enemies and let her fight them to see how long she'd survive (Master difficulty) and she took quite a while to go down. That being said, I lost her at one point, as companions have a horrible pathfinding when navigating cliffs and water. Normally she shows up at your house a few days later if you manage to ditch them and this was the case many times. However, this past time I lost her in the bogs outside of Solitude and now it's been over 15 hours of gameplay and she's long gone. Not at Whiterun or any house I own. I highly doubt she died to mud crabs out in the wild. I've since gone companionless from time to time and also have a new mage companion who I enjoy more - but damn me if I won't run into the poor girl again to regain my heavy set of Dragonbone

I'll kill her if I see her again just for a laugh ...