Better security for both the main site and sub / utility sites
Finer grained control over what users can access
Utility sites can be kept behind a firewall
Frankensites never get upgraded - they become security hazards!
More uptime for all sites
Individual sub-sites can be patched without bringing down any others
Fewer internal dependencies make patching easier
Easier patching means it goes faster, hence less downtime
Simplicity, Abstraction, and the Modern Website: 10 Years Later