I use the shields like this because they are really heal spells. They absorb all damage, therefore they work exactly like health, but health does not regenerate, and mana does.
With a permanent shield you can go through any windstorm area, and as long as you're careful, you can survive just fine with 1 health, since the health that you care about is in the shield, and it takes no damage from windstorms. This makes all windstorm levels the same, and you can farm some serious shards this way.
I try to use them the same way. I don't know if they need to be nerfed, but I wouldn't mind a second kind of shield, either. Perhaps a "generic" one that doesn't specialize in an element. In fact, here's a suggestion:
Add a generic shield that is meant to be sustainable, unlike the current ones which were meant to be burst. It would probably not offer a bonus against any element, to keep things simple. This new shield could have a set buffer amount of HP based on Tier, and would drain mana slower than the current one. However, it could also regenerate its HP (no toggling the shield to refresh it.)
The downside? The shield would drain extra mana while recharging, at a fairly high cost (10 mana per HP or something). And you cannot turn this feature off. The shield would also regenerate fairly slowly (maybe 10 seconds to get up to full HP). This causes two cases for the shield to fail. If you take a lot of burst damage, the shield crumples on you, and the extra damage left over goes to your HP. If you take a lot of small hits at once, your mana drops down to zero and your shield deactivates, leaving you vulnerable. These drawbacks are very similar to the current shields, but you're a bit less resistant to burst damage and a little more resilient to pea-shooter damage.
To buff the current shields for burst use, you could change the added resistance by element from 25% to say, 110%. This means you can actually heal from enemy attacks if you're tactical about it. Probably more of a novelty than anything else, but it would make boss fights more interesting.
This is quite a wall of text, and I know it belongs on Mantis, but I wanted to dump my thoughts out here. Off to repost it!