A 'sub-site' is an acs package, not just a term being used to describe 'an area of the main website adminstered by some (group of) users'.
You certainly could build yourself an intranet right now by mounting a new instance of the acs-subsite package at /intranet and making 'Herbert' the admin. Herbert would then create folders /intranet/bboard and /intranet/ticket-tracker and mount the appropriate packages. Then Herbert would fire up emacs and create an index.tcl / index.adp pair of pages that linked to bboard and ticket tracker and contained whatever else he needed and place them in the file systen at www/intranet.
An intranet *package* would be mounted at /intranet and would already contain the 'portal page' functionality Herbert just programmed and would automatically mount the bboard and ticket tracker packages, and would do alot more besides.
I don't know why a sub-site is 'cleaner'. A sub-site has specific functionality -- It provides a generic admin interface for mounting packages. ETP, Intranet, SDM are examples of packages with specific functionality that must be programmed, and may make use of existing packages by mounting them themselves.
