Quote Originally Posted by Uchiha Barles
I'll take a stab at it. At the very least, his sharingan are swappable. It seems obvious that each sharingan comes with its own set of ultimate abilities, so it makes sense that Danzou wants to catch em all! (TM) and be able to match the sharingan to the combat situation. I'm guessing they're embedded into his arm because it's a good way to keep them "fresh" as in, alive and usable, as well as a good way to quickly have access to them.
Also might explain why Ao didn't see them when he was examining Danzou's body at the summit meeting. If they're not active and don't have chakra running through them, they wouldn't show up as chakra points to a Byakugan user.