[kstrauser]

Your logic holds true for things like filesystem permissions and even separate user accounts. Since a privilege escalation exploit could give you root access, might as well do away with limited users and run everything as root to begin with, right?

And yet we do those things anyway. The idea is defense in depth, such that if one mechanism fails then hopefully another will mitigate the damage. Sandboxing isn't perfect, but it's another layer of security and I'd rather have it than not.