The drawback on Mills at deep safety is the speed. And considering he hasn't played the position in a couple of years, having the instincts to make up for it (like McCourty last year), would be critical.
Also, Marcus Jones still exists, and can certainly spot Jonathan Jones in the slot quite well. Jonathan might be starting slot, backup boundary, and if that's the plan, I'm more than ok with it.
Basically, I don't think we need another guy. That said, I'm really disappointed they didn't bother to upgrade Myles Bryant.