You can use parent and child portals to make it easy for adding other portal sites. Physically there is no difference between parent and child. But you can change child portal structure by adding an alias like www.domain.com to make it parent portal.
Lets say you have a host portal installed at www.hostportal.com
Child: www.hostportal.com/child
Parent: www.hostportal.com/child2
www.parentportal.com
It is easy to update their dnn versions when you have multiple portals at one hosting account.
But it is not easy to troubleshoot problems, because if host portal has an error about dnn, that error may effect all sub portals (including parent and childs)
I have www.mikroproje.com as host portal and www.mynetnuke.com as parent portal hosted by mikroproje.com DNN application.
I find it easy to use and manage under one dnn instance. But requirements are important. For example perfomance of portals.
Regards