Isolation is definitely important, but not all parts of the system running a single function need the same levels of access, and in fact it may be possible to target those components separately. Take a look at the wikipedia articles for 'defense in depth' or 'privilege separation' to see how important it is inside a system to treat each component isolated to itself as much as possible. (This is also why you don't want to rely on only a perimeter firewall for access control.)