There are some less obvious drawbacks (and possibly benefits) as well. With the ability, there's no way of making them stackable.
Because of this, and the limited permanent space (16 non shield/weapon), you can't have all that many of them in play at once.
It would also make protect artifact cover only one at a time.
On the other hand, they can hardly be considered pillars as such, so they'd be safe from earthquake (or you'd only lose one per quake).
I'm thinking a casting of 1 for the normal versions, 0 for the upgrades. One less quanta than a regular pillar, kind of.