On a multi-user, multiple virtualhost system, it would be nice if php3 scripts could "run" with the permissions of a different user for each site.  Apache supports this, with CGIs, via suexec.  This allows different customer's content to be completely isolated, while allowing them to create/modify their own files without being able to modify/delete other customer's data (it also does away with the need for 0777 directories, and the like).