Re: I dont get it...
yes you don't. I don't know what Oracle 12c offers but the multi tenant features I can think of all revolve around resource utilization. Sharing I/O, sharing CPU, sharing buffer cache, sharing DB connections etc. Don't want one user to be able to adversely affect another. With Oracle I think back to all of the outages I was a part of that were related to latch contention, that's a good way to kill all other tenants in a Oracle DB pretty quickly.
IMO has nothing to do with basic level security.
That doesn't stop most folks from running multi tenant databases and perhaps most of the times that strategy works fine (it has in my case for years on MySQL). But there are times when one app blows up and it causes issues with others on the DB.
Virtualization would be a more ideal way to go about it, since things are more partitioned, but obviously much less efficient from a hardware utilization standpoint.